I am going to be making a cake that looks like a stack of square presents for a small baby shower for my SIL, so 2 out of the 3 cakes will be dummies. Since the dummies have perfect, sharp edges I'm worried my real cake is going to stick out like a sore thumb. Does anyone have any suggestions to get those super sharp edges? Do you recommend bc or fondant on all? TIA!

I recommend chocolate ganache (sweet or semisweet but no white) instead.... way better to make the sharp edge you need

if you're covering in fondant put your cake into the freezer after it's iced nicely with square corners, then slightly wet the exterior with a damp paper towel and then put on your fondant, will help keep the edges nice and sharp.

If doing all buttercream, use a bench scraper and start at the front of the cake and push the icing straight back, then do the next side, and repeat until all sides are done and it will be perfectly straight.

Sharon Zambito shows this technique beautifully in her buttercream video

This helped me a lot when I first tried a square cake. Not sure if this is what you are looking for though.
http://cakecentral.com/articles/109/how-to-frost-a-square-cake

oh thank you all for your advice! lots to noodle on. I guess what it comes down to is my actual cake. When it comes out of the pans it's not super sharp, more rounded (I always use the bottom side as my top). Should I carve it to get the sharpness?
this is the only square cake I've done covered in fondant & it definitely has the rounded look I don't want-
http://cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1752118

check out this cake I did, this was the first cake I iced after watching Sharon's video on doing square cakes. I iced, squared off and did the impression mat on all 4 cakes in under 2 hrs. (which is REALLY fast for me, I'm known to spend that long icing a sheetcake)

I used fat daddio's square pans and they have rounded edges on them, so it makes no difference what your edges of your cake look like, it's all in the icing techniques.

If you're adding fondant, you can use ganache which will firm up nicely and give you a nice sharp cornered base for your fondant.

http://www.freewebs.com/khalstead/apps/photos/photo?photoid=75920794

thanks KHalstead! that really helps