When is "-less" used, and when is "-free" used?
Solution 1:
For me, the logic for when to use either suffix revolves around the denotation of the text you are writing. Use -less when a sentence refers to the absence of something formerly present, for example, a cordless phone. Use -free when a sentence refers to something that was never present, for example a carefree child.
I use the denotation because while -free tends to have positive connotations, -less can have either positive or negative connotations. But using the above rules based on context works in most situations.