If heat is a problem, you can just use the salt alone without heat. It will take longer, but it will work.
Ich is one of those diseases that can linger in a tank. Low levels of infestations (ie, in the gills, where it's not as visible) can then worsen if there's a stressful event. Treat, and treat for the full course plus 2 weeks past any visible sign. I prefer salt and heat to any medications, if the fish will tolerate it.
I've read to only treat for 10 days max
I recommend at least a full week after all visible signs have left. As OG stated, some could be lingering in areas not readily seen. IME, not unusual to see a salt and heat treatment extend over a period of 2-3 weeks.
Salt worked really well for me UN.
I read somewhere, can't remember exactly where, but that Ich is present in nearly all tanks. When a fish is no stressed and their immune system is running at full strength they can fight off the Ich. When the experience a stressful event or their immune system fails in someway they can contract Ich.
-hope that makes sense
Karl is right.It's often said, but it's not true. It sometime infects fish at a low level where it's not seen by the aquarist, but the ideas that it is present "in nearly all tanks", or that it has some kind of dormant phase, are nonsense.