Unless you have copper showing, there is no need to re tin. Tin will darken with use, and dark spots would be normal. I have a pot that just arrived from Rocky Mountain Retinning. After a couple uses, the lid is showing dark color. Especially on a lid, no harm done. The liquid hitting a lid is mostly steam. It is acids that will interact with copper.
1.5 MM is worth retinning. I personally prefer lighter copper in skillets as they are easier to toss.
Incidentally, RMR has a set of 4 3 mm saucepans in four sizes for around $350 w/lids. These have cast iron handles, including the lids, and are hand hammered. Peter assembles them from a bunch of parts he obtained a few years ago, and then tins on premises. These are among my best pots. Check his site.