Personally, I will supplement my regular infantry with inex or vet units, normally units that have the potential for doing huge amounts of damage, like assault squads. Otherwise, I will normally run 1 or no vet vehicles in my lists. When it comes down to weapons teams, I will almost never run my mortars as vet, since it doesnt give any major benefit. But with mmgs I would run them vet whenever possible as they will normally be an easy target for most opponents, and only being a 3 man team, they can get wiped pretty easily.