So, I know it's ideal to transport chilled cakes since they're more "solid", but there's always a risk of bubbles happening under buttercream when the cake comes back to room temp.... 

By that point, the cake is with the client and the thought of bubbles I can't fix anymore is very scary to me!

How do you all handle transport of a buttercream cake (both tiered and single) like that??

I've seen this in both chilled cakes and unchilled cakes. Best suggestion I've heard is to provide a "chimney" through the cake using a drinking straw under a pinhole for air to escape... but I've only used that method to coax bubbles overnight while I still had it and could cover the hole in the morning. Interested to see what others contribute here! Hate that!

keep the holes open is one way -- i just pin prick the icing through to the cake on each layer when i remember -- i put the holes in 'the back' -- I just try & hide them within the decor or something

Years back I never chilled my cakes but since about two years I do because it makes delivery so much easier. Never have a problem with bubbles. Think if you press harden ough on the cake after you filled it is the trick...or I'm just lucky...

Julia has the right idea.  I mentioned this in another thead regarding bubbles (aka: blowouts).  You most likely are not pressing the icing against the cake hard enough.  I don't mean so hard as to move the cake off the board but you need to anchor it against the cake enough to prevent air pockets.  Like Julia I almost never chilled my cakes and had very little of this problem.