Would you bid on ebay for an item that you knew someone on here was bidding for?

Is it worth having some sort of list of ebay id's of memeber that use it so you can tell if you are in a bidding war with a 'friend'

Obviously you can still make the decision to bid to the level you feel is right but (unless it breaches some law of ebay) it might make the difference of pushing the price up (esp scopes etc)