I think that we are in agreement that outside close combat, the only real source of anti-tank for the Daemon army is the bolt of change and the Soul Grinder. Discounting the Grinder, im struggling to find how many are needed to make a real impact on the army whilst not wasting points that can otherwise be taken on other units.
For example, i had a game yesterday against marines. I found that my 4 bolts (2 from Heralds and 2 from Princes) were quite ineffective. For the points invested in the 'bolt platforms', i could have bought a second GUO (the first one performed solidly as usual), and upgraded the princes to something more meaty (my usual are nurgle princes with Iron Hide, Nox Touch and Cloud of Flies) or just boosted my core units - though 3 units of plague bearers us usually enough for objecives capture.
Before i drop the bolts all together, i would like to see how many are usually used in other armies. Please also say what type of army you are running (Mixed, dedicated), because normally pure Tzeentch forces will have more bolts then others (at least i would hope so).