Depends on the tournament type, stack size(s), opponents, etc.
If it is a typical fast tournament, you'll want to keep playing aggressively to build your stack. In fact, in fast tournaments, there is barely a phase of the tournament that you can call "middle." Blinds are a threat to most stacks and there are usually only a few deep stacks.
In slower tournaments, you have some wiggle room to play "real" poker, read opponents, and make a few moves without risking your tournament life. You just can't ignore the blinds/antes, though. If they start gouging large portions of your stack, you need to move fast.