You won't have any problems riding the bike with the pedals creaking, the BB will just get destroyed in the next 1000km or so. Better to get it serviced straight away, while it's still possibly repairable.

With Shimano stuff its almost always the right cup working loose and it's easy to fix: just have to pull the whole crank assembly apart to get to it And once you get it it keeps coming back more and more frequently. I only bother the LBS once with this problem before shoving a new BB assembly in. Pay as much or as little as you want. 105 BB is just as good as Ultegra for training/general riding IMHO.