I must agree with Berlichtingen, hold them back and keep them moving once committed.
The standard german tactic when on the advance, was to hold them back about 1000 yards behind the advancing infantry. When the infantry were held up by anything too hard or time consuming for them to deal with, the Stug's would dash foward and overwhelm the enemy battle position/AFV with sheer weight of fire. Hence "Assault gun".
When applied to combat mission, it is important to note that the above tactic makes two assumptions:
1. Shock value is largely dependant on a combination of surprise and superiorty of numbers. So take a few of them, three is a good number. Do your best to conceal them from the enemy until you need them. When you do bring them out don't pussyfoot around, hit them hard.
2. The advancing infantry have positively identified the enemy threat and its location. In game terms this means that you should have gathered a good picture of the pieces in play before commiting your Stug's.
Lastly, work in concert with the infantry, this cannot be stressed enough. It's often useful to have a platoon embarked with the Stug's when using them in this role. You'll look hard as nails aswell!