It's all depends upon you availability of TCredits.
You will have to guess things.
Just set the limit in Auto-bidder to check the response, you will come to know.
Both techniques are very useful depends on the time and your interest in auction.
If you badly need it and you have enough TCredits in pocket then go for Auto-bidder.
But if you want to play smart and wants to save some Tcredits then go for manual bidding.
