I would say once the cake is delivered and on the table, that way you ensure they're as "fresh" as possible so they won't wilt and also because real flowers are often much heavier than silks and once they're in the cake and you move and jossle the cake around you have a greater chance at ripping the cake apart.
To keep the flowers as fresh as possible, I place them during set up at the venue.
Be sure to have enough of the little plastic thingees (sorry, a bit of old timers there) to place the stems in.
I have done a lot of fresh flower cakes and I can tell you that it all depends on the type of flowers. Each flower has a "cut-life". The time it can last after being cut.
It also depends on if you are making an arrangement and using floral foam or posey stick or just laying them on the cake.
I usually have ALL arrangements made and on the cake the night before the wedding. Unless cut-life is extremely small, 4-5 hours, like hydrangeas, daylilies, or magnolias.
I would suggest calling a local florist with a list of the flowers you will be using, and ask them the cut-life of them. or if you have time, experiment with them yourself.
One other thing, make sure you check that the flowers are not poisionous. 
Thanks for that info!
Also, the bride wants the flowers trailing down the cake, and I was reading online about using florists tape and wrapping around the stems so you can poke them into the cake.....is this ok?? Does it work ok??
I agree with all the other comments but just wanted to add please don't just wrap the stems and stick straight in the cake always use a posy pick. You can wrap the stems together before putting them in the pick. You can get all different sizes hth 
Thanks for that info!
Also, the bride wants the flowers trailing down the cake, and I was reading online about using florists tape and wrapping around the stems so you can poke them into the cake.....is this ok?? Does it work ok??
That is a no-no. If some one is alergic to latex, which is in floral tape, which is why it works so well, you could KILL them.
You dont want to serve any guests anaphalyatic shock cake without an epipen chaser. 
The straws work great. you can use large, small, coffe sirrs, etc.. to fit your flowers, you can wrap in floral tape then insert that into the straw. Or you can insert a few flowers into the straw to make bundles, you can also fill the straw with royal icing to hold the flower in place inside it. Just a few ideas for you.
I always place flowers at the venue. I'm too afraid of flowers wilting or turning brown. It works out too because should any little mistakes pop up, they're covered with flowers! 
Wrapping the flowers in floral tape will help to extend the "cut life" or how long they stay fresh. It does depend on they type of flower, each has a different cut life.
Roses keep very well when wrapped and so do lilies and orchids. (think of flowers you usually see in corsages, they are the ones with the better cut life) You can wrap them a couple of days before hand and keep them in a airtight container in the fridge. Just mist them with water from a spray bottle at a couple times to keep them fresh.
I would use a flower spike or a straw to insert them in the cake. It is more food safe and it helps support the weight of the flowers on the cake.
Please explain little about using straws. Do you just cut the straws the length you need, insert the flowers and insert the straw into the cake below the icing level so you don't see it? I'm sorry for the questions, I've never used the straws.
Yep, that's pretty much how you do it I like to cut my stems really sort and insert a wire in the base of the flower (where the stem starts) and then cover both the stem and wire with floral tape because then it is less bulky. Then just the thin wire goes in the straw instead of the bulky stem and you can fit several flowers in one straw. Sometimes it helps to put royal icing in the straw to help "glue" the flowers in place, but that isn't always necessary, it just depends on your design.
You can also wrap your flowers together into groups with the floral tape before putting on the cake. Wrap each flower/ leaf individually first and then you can wrap them together adding one at a time. This can help you make a cascading effect without needing to have very many straws or flower picks in the cake.
Please explain little about using straws. Do you just cut the straws the length you need, insert the flowers and insert the straw into the cake below the icing level so you don't see it? I'm sorry for the questions, I've never used the straws.
You can also take a lighter and burn the end of the straw slightly and pinch it together...it will form a seal and then you're guaranteed when you poke it into the cake that no cake will come in contact with the flower stems/florist tape, etc.
dipping the flower stem in a little melted chocolate and then putting inside the straw works well too (I do that mostly with greenery since the stems are really thin most times), most flower stems fit nice and snug inside a small drinking straw though.
You can buy wedding bouquets online to make your wedding a memorable one. Different varieties of flower baskets are also available at reliable prices, so start shopping now.
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