No, you can use as a personal challenge, but it's not a rule, maybe depending of how your game will be, it's important. But many games in the past jams don't have 8x8 grids. And the only rules you have to follow is the original jam rules. A good example is the games who won the gbjam3 and gbjam4, both are awesome, and they use more than 8x8px, maybe more than 16x16.