when you perform a deadlift with resistance bands, if the band is thick and stretched a lot(which is possible since it is a compound movement and you are going to have lots of power), you might elevate the band underneath your feet and lose balance.
resistance bands help people who lack flexibility and overall strength and mind-muscle connection(especially with compound movements) to do deadlift properly. the band should be kept in the center of the feet, no matter what variation you are doing(conventional, sumo, stiff-legged, and etc).
the way you should position your body is almost the same as barbell variation just the band is going to be closer to your body compared to a bar. that is going to put the emphasis more on the upper traps rather than the lower back. also with a band, you have less stretch and tension on hamstrings but your glutes are going to be activated more.
make sure you keep the head aligned with your chest and do not bend it forward or back. also, make sure you do not bend the elbow at all(it can cause problems in your biceps and elbow if kept bent). you have a little bit of restriction if you want to bring legs closer to each other(shoulder-width apart or even closer).
the reason is stability. for that matter, it is advised to take a wide stance and point the toes out a little bit while doing deadlift with bands.