I've always been taught that when doing resistencia you should stay flat footed, but with passa a frente to go onto the balls of your feet and to have your knees closer together. Is there any practical reason why one should stay flat footed (or vice versa)?
Heh. Interestingly enough, where I was taught, the primary difference between negativa and resistência was the foot positioning with resistência being the flatfooted one. Just goes to show that the terminology varies a lot between schools.
I'm not familiar with passa de frente, but I assume it is something like a queda de quatro as seen in this video. In that case, you're on the balls of your feet because your weight is over your feet, which can't really be set flat in the movement without breaking your ankles. You could do a similar movement flatfooted, but the weight distribution would be different, more split between your hands and your feet, with your feet further out.
From there, I'd probably compare it similarly to how negativa and resistência are used differently. The former is primarily a legs movement, moving down and back into the esquiva. One hand is often used for balance, but the primary weight is on the supporting foot, which is flat to let you use the large muscles of your leg to absorb the downward force. The latter tends to be more often used while sliding the extended leg out, using your hands for support and weight distribution while moving forward. Thus, you are on the ball of your supporting foot because a) you're supporting less weight and b) your center of gravity (your butt really) is pushing more forward, above your foot instead of behind it, requiring a different foot position to put less strain on your ankle.
On a side note, I have since learned that resistência is also used in some schools to describe the basic esquiva of leaning backwards to avoid a kick while moving the hips forward to counter-balance, staying on the feet, then coming back up, as per the capoeirstra on the left in this image:
In that case, one generally goes up on the toes because otherwise your center of balance is behind you, and you're going to continue to fall backwards into a queda (and indeed, some groups do just that).
Ultimately, your best bet is to talk to your instructor if they're available.