Agree with DorianGrey's std SU assignments, but for sure PzKorps for me have same assignments. Like to have 210mmH's & 150mmG's and any nebels assigned when possible. But bigger arty than these I reserve to AG level. Use only at prepared seiges. (Lgrad, Moscow, Kiev, etc.) Various units like MG batts, Bicycles, flamePZ's, etc. I reserve again at AGs. Assign these to weak infantry as needed. (I like force balance.) Do like assigning excess SU's to specific units when possible and available, too. All for force balance.
Not sure by practice or design (manual not clear to me) if more than a few SU batts are ever committed or if over-assignment gives value through multiple attack/defense in one turn. BTW, are other than Korps level units ever committed?
Outside of points-to-assign, the cost of over-assignment is not clear. There is some distance constraint in the relationship of available SU's and target SU's, but not so clear to me. Seems SU batts can be flipped all over much of the map w/o fatigue, rail, losses, or chain-of-command effect. Do roving air units ever nail these SU's?
Would appreciate experts' thoughts on issues for SU's.