I disagree with selling your common souls, and here is why:

Common souls = Ironite

How so? When you sacrifice 10x 2* characters you’ve got a decent chance of getting SoT, and occasionally a few actual ironite. If you open a stack of 99x regular souls, you’re going to get at least 10 shots at gaining SoT or Ironite. The SoT usually comes in anywhere from 4 to 10, and Ironite usually 4-9 (don’t quote me, as I’m going off memory). Considering a SoT refill costs you 30 Ironite, getting upwards of 40-60 SoT and maybe 5-10 Ironite out of a stack of common souls is a hell of a lot better than a few thousand gold. One run in the newest dungeon (can’t think of the friggin name, but you all know what I’m talking about) will give you 14-18k in gold and only cost you 8 SoT. Besides, sacrificing a few stacks of common souls will also make a noticeable dent in your vortex — not a ton, but an eighth at least.

tl;dr

Sac your common souls, it’s worth the strain on your thumb.