You need to know what your team are capable of. If you have bowlers who can bowl areas when asked to. If your opening bowlers can bowl full and outside off stump, then it may make it easier for you setting fields. Yes in T20 most will go for runs, but as stated previously, you need a plan and for that plan to work, team mates will need to be in on the act also, not just the bowler, captain and keeper.
You also need to know if your teams mates can field close or are boundary fielders or ones you need to hide.
As a captain you need a good view of the game to survey what is happening and if changes need to be made.
Remember, if things are not going your way, it is not solely down to you, it may be poor bowling or just that the batsman is decent and in good nick.