It always helps to be below frost level to avoid frost heave. I would do it if it was me. It can move your deck out of level but the effect is hard to guage with an independent deck. It may move some if you don't. At least go as deep as the others if the price of longer posts is out of your budget or you get tired of digging.
Around here it is a good idea to dig a bell shaped hole to help avoid frost heave. You see lots of poles with V shaped concrete slowly pushing upwards above the ground. Good luck.