Keep in mind that offers come and can go, so if you do have an offer and receive a better one that you want to take, I wouldn't be in a rush to give up your first offer. Things change in corporate, and they really don't have any loyalty to you yet, so I would cover all of my bases.
I wouldn't wait until the last minute to resign an offer, but I wouldn't do it 4 months out from a start date either. This is business, they know that and also know you will continue to receive more offers as time passes. I am sure they receive quite a few offers returned to them, and they may even hire based on those expectations, and if for some reason they don't get any offers returned, they just may cut your offer. Turning down an offer from the start isn't that much better than accepting one and then rejecting it later on, either way you would be burning a bridge in a way..."oh NOW you want to work for us?". I'm not sure that offer would be the same if you had to go crawling back, so make sure you don't have to.
If it was an indepenent, handle it differently.