Well, if you bid the most you are willing to pay, when  you get 
outbid you can just sit back and say "Well, I wasn't willing to 
pay that much anyway." Sometimes in the heat of the moment at 3am
when you have just been outbid with thirty seconds left to go, you 
might find yourself typing in a number you would never have considered
for the cel in question.
I always find it a little bit hard on the blood pressure to watch 
an auction skyrocket hundreds of dollars in the last two or three minutes.
For my peace of mind I have no qualms going ahead and bidding my max from 
the outset, even though it drives the price up initially. It 
prevents the sort of 'end frenzy' that goes from $40 to $150 in the 
space of under sixty seconds. If it's already /at/ $130, then toward the 
end, it's probably only going to go up a small proportion rather than a huge one.
I think it's all a matter of personal taste and how much stress you can deal with 
versus how much you want the cel.
I have watched cels I /really/ wanted slip through my fingers in the last moments, 
but contented myself with the knowledge that I had stated how much I was willing to 
pay and stuck by that. At a certain point you just have to let go. ^_^
Ryochan