Who knows what tactics are right

The Smart guns that Drake and Vasquez are armed with are support weapons in that they are heavier and built for rapid rate of fire, depending on the situation who knows whether or not these should be at the front or not, though why they're point once they've supposedly handed in their 'chips' is another story (or nitpick).

Heat tracker

The smartgun (made from german WW2 mg42 machine-gun) tracks movement and the gunner has heat vision so it would make sense to send the man with the best vision into the dark.

Following the Heat Tracker comment

THe M56 smart gun is usable in a variety of roles. However, a comparison of the M56 to any modern day machine gun, including the US M60 machine gun, is a mistake. Starting off, the sensor equipment allows the operator to see in Infrared and enhanced, night vision. Slaved to this suite is a power hydraulic system that, when engaged, brings the weapon around to track the target. Once the firing stud is depressed, the weapon fires and continues to track the target until termination, which, by the computers estimation, involves the target being reduced strcuk so many times or reduced to enough pieces to be determined neutralized. Also, might I add, the sensor suite includes an apature cone for motion dection (much simpler than the tracking models carried) which helps follow the target when the muzzle flash "whites out" the target (according to source manuals, this is somewhat common in high heat areas). Thus, given these charateristics, a weapon with its rate of fire and butal accuracy with the support of a motion tracker should be up front where the dilivery of high-velocity fire would be most effective without allies in the way.

Think about this though...

Part of the M56 use in Aliens is a heavey armor plate front mounted on the person using the weapon. This, to me, would suggest that it IS an assault type weapon and that Vazqez and Drake probably chose not to wear additional head armor (i.e. helmet) that would likely normally be a second part of their gear.