Halvarez almost never bids much, in my experience. He's like a vacuum that just sucks up tons of names at the default prices. Sometimes he'll bump his bids all to $76 or so at the last minute. But I think he is in too many auctions to worry about individual names much.
Saggydimes (BuyDomains), on the other hand, can be trouble. They have plenty of money, but are pretty disciplined. So if it is a domain you really want, and you're willing to pay a bit over "market value", you should be able to win it even if they go after it. And sometimes they don't really care much about the domain and won't bother bidding more anyway.