Choice is often offered (or would likely be agreed to if asked) if it's a bunch of pretty good stuff.
If it's a bunch of pretty good stuff, and choice isn't offered, the auctioneer may be willing to sell choice if he's asked. You can bid up to what you want to pay for the lesser value things and stop. If you are high bidder, you really made out. Likely you're not, so once the bidding ends, you are risking that the winner doesn't want them all for x-times the price. If he doesn't, then you can start bidding up to your price again on the remaining. If you really want the lesser stuff, I think you'd be better off to ask for the lesser stuff split from the lot and bid on first.
In the case where it's some good stuff with a bunch of "bad", the auctioneer probably grouped it that way to move the "bad". He may be willing to go with choice, and it doesn't hurt to ask, but I don't think he's going to agree. He's going to naturally think that you and everyone else is interested in the good stuff. His experience tells him that if he sells choice, the price will run up and the winner will take the good stuff. And he's left trying to sell the "bad" that no one may want. I think your chances are going to be better if you let him know you're interested in the "bad" stuff and ask if that can be split off and auctioned first.