Do any of you trim the sides of your cakes before you put on the
fondant? Or do you even it up with buttercream. I've noticed if I
just use buttercream the sides aren't perfectly straight.

Not sure I understand your question.

I can offer you this tip: fill the layers and LET THE CAKE SIT before adding fondant. This will prevent that funky bubbling under the fondant between the layers.

Duh, never thought about that. Thank you. Sometimes when I bake a cake
they don't come out exactly the same size. So when I stack them
there not the same width. Hope this made a little more sense.

Thanks again

For basic shaped cakes, I often cut my cake board base the exact width and shape I want my final cake to be. Generally the base is 1/4 inch wider than the cake itself on all sides.

I then ice the cake with bc building out on the sides just a little bit beyond the cake board base. Finally I take a wide putty scraper and place it perpendicular to the base of the cake and run it along the base to smooth the icing.

You will end up with the sides of the cake being perfectly perpendicular. Any bumps and bulges disappear during this process. That's how I do it. HTH

I trim my cakes before I frost them if they are not all the same...everything shows up underneath fondant. When I first started out with fondant, I would try and fill the valleys and gullys with frosting...but that didnt work cuz the fondant sank (in the valleys and gullys
A lesson well learned...oh well....

That is great information, but when you trim the cakes. How do you prevent from lots of crumb or what do you do for crumb control?

...if there is an excess of crumbs, I brush them off with a pastry brush...then the frosting covers up most of them...I frost once (crumb coat), then frost again then fondant....the crumb coat is really thin...ya know what I mean?