Situation. after the 3rd hour, this is very comon to all daily live tournaments running around vegas.

blinds are 400/800. your stack is 15k. average stack is around the same.

blind levels are 30 mins.

a couple of the players at the table are short stacked. about 4-6k, and 1 big stack at your table with about 40k

do you tighten up and wait for a big hand to pick up chips with?

play more aggressive and steal vs. weak players, or average sized stacks?

or

play tight, but try to keep your stack at average by stealing and waiting for a big hand?

I think it depends on the table itself, and the type of players you're playing. If you do become aggressive, make sure to target those that will pay you off. If you're image was tight before this situation, maybe you can bluff people out of pots. I normally tend to be tight, waiting for a big hand while I still have a decent chip stack. Avoid confrontations this early in the tournament, unless I'm in w/a big hand PF.

Probably what I would do because TAG is my standard game

I think the decision is entirely situational.

With an average stack, i'd just play averagely. Don't think about it as LAG and TAG, think about what the situation of the hand dictates, would it be profitable to make a steal at the weak tight blinds when it's folded to you in the CO? If so, raise, if not, fold. When you're average in chips, i really think the best idea is to just play the situation and forget about how LAG/TAG you're trying to be.

If you were the chip leader or had a really massive stack i think you have a big advantage over the other players and should use it by being more aggressive than you would normaly be. Some people say it's better to play normaly, but imo if you have more chips than everyone at your table it's a mistake not to use it to your advantage.