I'm currently making some dummy cakes to use for wedding shows etc. Can anyone tell me when stacking do I still need to put dowels in for support

No, if you're using styrofoam dummies you do not need to dowel them. The styrofoam is strong enough to support all the weight. Just use a dab of frosting (or even hot glue, lol) to "glue" the dummy tiers together.

Thanks I thought they might be okay but wanted to check

If you finish each dummy of as an 'individual' cake... nicely finished base/border...then if you do not 'glue' them together you can re-arrange them in different ways to show different setups. Like mixing rounds with squares or heart shapes. Just make sure each base border is nicely finished and will sit smoothly on the top of another larger cake.

Same with the flower accents....if you use gumpaste flowers or even silk flowers, you can move them around on the display cakes to create another look with the same cake.