Unfortunately, there is no correlation between Snails and String Algae (which favors cooler climates).
True, there is nothing wrong with Ramshorn, Apple, or Trapdoor snails. They are insignificant scavengers. However, what Cambria Koi has discovered are the eggs of the Great Pond Snail, Limnaea stagnalis. These are not good snails. They can spread quickly (multiply like bunnies), choke up filtration, and are intermediate hosts of certain parasites (anchor worm). As they complete their life cycles and die, the dead ones smell pretty bad, and decaying snails can mess up water quality.
I've found that I can't really get rid of them, but I can keep their populations somewhat in check by constantly wiping off the eggs from the undersides of the lily pads, and tossing any that I find in the filters, pond, or pots. I can toss dozens at a time from one Lotus pot.