Riflemen: if a squad has kills and is faced with a force they will not live through, I will usually retreat them.
I try to keep most of my units alive. I cycle through reinforcements like this. Bring on a few squads, get a bunch of kills, and retreat them if they are overrun. I rarely ever retreat vet0 squads though unless they have achieved a substantial amount of kills.
The only time I've truly lost my vetted units is when I make silly mistakes (like running a Sherman headlong into the enemy line).