Best to have an idea of what it is worth and know what other people will likely think it is worth.
If you are a reseller bidding with other resellers then if you just surpass them you could do quite well, however I would not even compete in an end user auction as domains then tend to go way beyond any sense.
If a domain can be sold for $2k then why would I be in a room bidding on a domain for $2k? I might bid $200-300 but that is about it.
Take emotion out of the equation, and set a number then stick to it, the worst thing I see people doing particularly in real world auctions is keep on bidding against someone else just to beat them.
The idea is to resell it, so if you cannot get it for the price you want then walk away and put your money elsewhere.
For the reasons above I would not even start bidding on auctions until you can recognise a domain that is undervalued consistently...just start off making mock bids on what you figure the price will go to and see how accurate you are, it will be the best educational tool that you will find for this purpose.