What does the rule of thumb even say? How high should they bid when they see an attractive lot?

If your rule of thumb says that you should only start bidding after a third of the auctions are finished, I hope you see that there's a pretty obvious way for others to arbitrage against you.

Auction theory studies both optimal bids and optimal auction designs - imho it's one of the more useful areas of economics: https://en.wikipedia.org/wiki/Auction_theory