Everyone has thier own opinion on this one ranging from no SU support beyond what you get at the start to heavy SU support. You have to get a feel for what works for your strategy.
The singlest biggest bang for your buck will be sapper regiments. They dig, they destroy fortifications, they dig, and they help on offensive and defense. Did I mention they dig? $ for $, they're worth more than all the rest of the SUs.
As far as what I do, mine runs to the following for the SU initially:
a) Disband all the MC regiments, AT units, fortification zones, PVO/AA battalions, sapper battalions. The MC regiments simply require too many trucks for the CV; the AT and FZ provide a lot of guns; and the sapper battalions are useless in my opinion. The AA battalions don't provide that much support.
b) For each front, I generally build 1 PVO regiment and two RR construction units. (Later, in 42 when the manpower situation becomes better, I up this to 4 RR units).
c) For each army, I want two artillery regiments - and I prefer RVKG units due to the mix of calibers, a 36 gun 120mm battalion, plus 3 sapper regiments and on PVO regiment.
d) For each air HQ, I will assign either 1 or 2 PVO regiments depending on how I feel. I tend to go with two but that because I've been burned by the German air force in other games.
The PVO regiments are worth thier weight in gold - these units will kill more German planes that your airforce will in the first year to 18 months.
Later on, I start to increase the RR units and will add another 41 mortar battalion (cheap Arm pts and manpower). That's usually in 42.
As far as what to do on turn one, I set ALL HQs to zero support units and let SU migrate to STAVKA (set on 9 SU level). It takes about three turns and a few engineer units won't ever migrate. Then you do a grand railroad tour around the army HQs and fronts to distribute the SU. You usually have enough to provide each army HQ with 2 artillery units. Mtrs have to be built (and I usually disband the hvy Mtrs). Over time, as I have extra AP points, I convert these to RKVG units. I don't get the feel that the heavy only artillery units actually pay off defensively. They use a lot of ammo up fast and don't seem to cause a lot of defensive losses.
Later on, in artillery divisions, the heavy guns on offense are real killers. But defensively, they just haven't performed as well for me. That said, I do like the mixed RKVG regiments from a supply perspective AND the mixed range you get.
On the other hand, several very good players don't assign artillery units to Armies/Fronts until close to the first blizzard. They feel that the SUs take losses and don't provide good value simply because the Soviets are going to take a pounding no matter what you do. I would just rather cause as many losses to the German as possible.
I avoid the light cannon units (76mm SUs). For the shock armies, where I put my cav divs that turn into corps, I will usually add two or three heavy tank regiments.
In 42, if I have a lot of supplies, manpower and APs after the Soviet TOE changes, I start adding tank regiments to the army HQs. And I tend to standardize all my armies into infantry, cavalry or armor with the same support for each.
In 42, when all the Artillery Divisions are avaliable, that's where the guns go. And I don't build artillery BGs. Too light and I haven't gotten a good bang for the buck.
Anyway, my 2 cents.