Unless you decide to shut that register off completely, you are going to have an opening to vent that heat regardless of weather it is front or back. How do you want that heat to travel? Under the tank and into the room, or behind the tank and up the wall? Just like your tank needs flow for all the inhabitants to be happy, so does your room. Air flow, water flow.. same thing physically.
Structurally, a small opening the size of the heater register, 2 x 6? 3 x 12? Won't impact your stand integrity as long as you reinforce the area where the opening will be. If you decide to run it out the front, you can place a diffuser of the front of the stand to make it look better than an open hole. There are unfinished hardwood diffusers available that can be painted or stained to match your stand so they will look more integrated.