I know that the safest way to use fresh flowers on a cake is to use organic non-toxic flowers and wrap the stem with floral tape and/or insert using straws. But I'm curious to know how you do the cascade of flowers like the picture below. If you're using straws wouldn't that make too many holes and possibly ruin the structure of the cake? Is it ok to directly insert the floral taped flowers on the cake without the straws?

Also, what are some methods for placing just regular (non-organic but non-toxic) flowers on the cake? 

Thank you in advance! 

I know of one decorator who was also a florist and what she did was pipe a huge swag of b'cream icing where the flowers would sit then she claimed she was the only one who would cut/serve that cake so no icing used to held the flowers was ever served.    Another way might be to do something similar but put down a small swag/trail of icing, & cover it w/some plastic wrap so the flowers don't touch the 'main' part of the icing.  Hope that gives you some idea of how to proceed.

I've never used fresh flowers on a cake, but I see no reason they could not be attached the same way as the sugar flowers.  When I have a lot of flowers and I want them to flow around the cake, I wire together a posy about 12 inches long and the whole posy is positioned in one large straw.  Line up the posies so they appear to be a continuous cascade.

I use fresh flowers all the time. I like to place my flowers individually, so I have a bit more control over the whole look of the thing as I go, rather than creating posies (not that there's anything wrong with posies!! I just can't do 'em!). My style is a little less structured than a posy. I wash each flower and wrap each individual stem in parafilm so there's a watertight barrier between the stem and my cake. Don't want any nasties getting in! I insert as short an amount of stem as I possibly can, without it being so short that it might fall out. And quite often I'll just sort of 'pin' it to the side of the cake with buttercream and not insert anything at all. For a cascade, you can kind of use the flowers at the bottom as a sort of bridge/platform/support thing for the flower above it (I hope that makes sense).

Hope that helps!