When should I sell towers?

There's probably a reason we're given the option to sell towers in Squadron TD, but I never sell them because I don't wish to waste what I've spent.

For instance, after wave 20, is a Dark Mage worth keeping? Or is 50% of his paltry value worth putting towards yet more Lords of Death?

How can I determine when I should sell towers, if ever?

Solution 1:

I sell towers pretty frequently if I need just a little more to get me to the tower I need. I like the custom builder and use zerglings, so it's not a huge issue.

But usually I don't sell units because they're always good to take at least one hit from a wave before moving on to my better towers. It all depends on your build.

Also what if you miss click a tower? There's no other way to remove it (that's why it gives full cash back if you do it in the same round.) But yah, it's mainly used as an "oh shit" I need a lil more to get the tower I really need.