Against 3 limpers with random hands, you're about a 4.5 to 1 dog if you're holding 74o. Against 2 limpers with random hands, you're about a 2.3 to 1 dog with the same holding. If you haven't tried it, there's a great tool called Pokerstove you can download to run these kinds of scenarios (search the forum or google it). If the pot gives you better odds than this, you can complete.
You can also look at implied odds - the odds that your hand will allow you to get more money off your opponent than is actually in the pot, if you hit with a sub-premium holding (because people will be disinclined to believe you completed with 74o).
You have to be careful with pot odds
and implied odds in the SB, though - your position is the worst possible, so you typically won't be able to extract as much value from here as you would from good position. As JoeDi points out above, since you're first to act and don't know if your pair of 7's is outkicked, your plan of attack from SB is difficult to map out. You will only get good value from two pair or better with 74o - and that's assuming someone else hit something on the board that isn't better than what you hit. Because of this, I tend to look for at least one and a half times the pot odds to complete from SB with complete junk, more if I think the table has several good players. Suited connectors and small pairs I'll complete when I have pot odds (especially if I think I have implied odds), though if I like my situation I'll be inclined to raise those rather than simply complete.