Generally, one will cut off the unwanted seedlings once at least one of the seedlings in the group has 2-3 real leaves showing. If there is a good reason to cut the culls off before that (for example, some of the seedlings are much weaker than the others, or diseased or damaged or so many have sprouted in the same cell that there is no room to grow) then go ahead, but try to leave a couple or three viable plants in the group until they have grown enough leaves to be able to tell the winners from the losers. The reason for this is if you cut too soon and then something happens to the remaining seedling, then you will have nothing to transplant from that set.
Btw: I say "cutting" above because cutting the culls at the soil line or just above is less disturbing to the remaining seedling(s) than pulling them out by the roots.