[ChrisV]

Disclaimer: The following applies more to higher buyin SNGs, where there are more aggressive players. If you're an experienced player you probably know everything in this post already, but it doesn't hurt to get concepts clear in your mind.

When is the best time to steal from your opponents? Most players understand that in allin steal scenarios, it's better for your opponent to have a smaller stack. If he has a large stack, applying ICM concepts will give him similar results to just using pot odds, and he'll just call whenever he thinks his hand is probably better than yours. The shorter your opponent's stack, the better, all the way down to the point where the blinds start becoming such a large percentage of his stack that he's forced to call loose.

What about non allin steals? Once again, it isn't a good idea to steal from very large stacks which can't be hurt by you. This time though, I don't think it's a linear progression down through smaller stacks.

Consider the typical steal situation:

Hero (1900)
SB (irrelevant)
Villain (???)
Blinds 50/100
Dealt to Hero [ Qh Js ]
(folds to Hero)
Hero raises (300)
SB folds

What stack size would you prefer Villain has? In this situation it's unlikely that he has enough that your 1900 can't seriously hurt him. Let's consider stack sizes from, say, 1200 to 2400. I'm pretty sure everyone agrees that we can't really call a reraise for any of those amounts. Villain calling is outside the scope of this post (as I said at the top, I'm assuming aggressive opponents), but I think everyone will agree it's nowhere near as bad as him reraising.

Pretty clearly, any reraise from the villain will have to be allin. If he has a mediocre hand, say AT suited, the object of his reraise is primary as a resteal, to make you fold. Since we can't call a reraise no matter the amount, villain is better off with the stack of 1200 than with any other amount. It accomplishes his goal while putting much less at risk than if he has a stack of 1900. I refer to this as stack efficiency. Villain has maximum stack efficiency at the point where you will fold steal hands to a reraise. Minimum is at the largest stack size he has where a reraise still has to set him allin. It rises again if his stack is larger than the effective stack (ie when he has more chips than you do).

So the original question: When should you steal from your opponents? You should be more inclined to steal when your opponent has low stack efficiency and less inclined to steal when he has high stack efficiency. This will leave you less vulnerable to resteals.

Also, when you actually have a good hand, if you don't think your opponents will be tipped off by bet size variation, you can change your bet size to give them maximum stack efficiency. For example, if you and your opponent both have a stack size of 1700 at 50/100, you might try a raise to 400 instead of to 300. This also has the effect of looking like a donk attempt to prevent a call, but against an aggressive opponent it will actually encourage them to attack you.