Question:
Isn't the actin polymerization buffer (being 50 mM KCl) considered low salt, and so then shouldn't it prevent His-tag aggregation?

Answer:
His tag aggregation appears to be a hydrophobic phenomenon because it is reduced when you lower the salt. At least thats our experience. We have used just 10 mM Pipes buffer in order to minimize the aggregation of some his-tagged proteins. 50 mM is low salt in a lot of applications but in actin work, that would be considered medium, low would be 5 mM Tris.