This is the point of view from the districts I have taught in. Cash is always viewed as a bribe for grades. It is sad to say that there have been some teachers, parents and students who have actually been part of this. There is a greater "paper trail" with a gift card so less chance of it being seen as one.
I know some schools send out a teacher "bio" at the beginning of the year. Not all do this although for teachers with small numbers of students it can be really nice. I had 180+ students on my roster on any given day. You can imagine what kind of crazy that can be if we did something like that. I teach older students and gifts are fewer. For some reason, parents think that teachers of older children don't need appreciation. Whatever you do, I think a note from the student is always appreciated. My most treasured gift was a letter from a "problem" student who thanked me for giving him hell for 36 weeks and not giving up. Twelve years later, that note is still preserved in my special portfolio.