There are two different uses for heat when treating ich.
It can be used to speed up the life cycle so that the salt treatment works faster (generally 80-82 degrees is reccomended for this), or it can be used by itself to kill the ich (it doesn't actually kill it, just prevents it from reproducing), typically temperatures in the range of 86-88 are used for this. 90 is a touch on the high side, but it's not gonna be lethal in the short term.
I would keep you temperature up for a week after the last signs of ich dissappear, but opinions vary on this (make sure it's at least 3 days).
Personally I use a lower heat and table salt, but either method should work just fine.
http://www.cichlid-forum.com/articles/ich.php Has some great things to say about ich treatment and prevention.
~Josh