It is best when doing such repairs as water pump or radiator to change the thermostate at this time. I prefer the benz coolant over any other. Also make sure the auxilary fans are working with the air on. In your case you said temp stayed same with air or not, I would look at doing the thermostate for insurance, 90 on a warm day idling isnt too far off the scale but I would rather have a known good thermostate than have one leave me stranded.
