The great thing about most acrylic is that there's not really a front/back! While the front and back can differ on some sheets, your creativity has no boundaries when it comes to which side YOU want to use.
With that said, there are some industry standard practices on a few materials.
First, Mirror Acrylic almost always has a gray backing (this is what makes it mirrored)! Most people consider the gray backing to be the back, and prefer to engrave mirror with the gray backing side up and the front (transparent/mirror side) down on the honeycomb tray.
For printed or patterned acrylic, the rule of thumb is usually that the front (pattern) side is face up in your laser and the masked back side is face down. This gets tricky If you have both sides printed. In that case you'll usually want to raise your acrylic off the bed of your tray.