Is the air leaving cooling equipment always considered to be at 90% RH?
Yes, even after condensing moisture out of the air, the cold air has contracted (think ice cube on a t*tty) so the moisture that is left takes up a "relative" high amount of
space in the cold air - the supply air has moved close to
the Saturation Point Curve of the Psychrometric Chart.
From ACCA Manual P (First Edition) pg. 29 it says "In actual practice, a supply air condition which falls on the RSHR (Room Sensible Heat Ratio) line and has a relative humidity between 85% and 95% is a good choice". Also, "A supply condition that falls below the RSHR line can be used, but the room humidity will be lower than the desired design humidity and If a supply condition that falls above the RSHR line is used, the room humidity will be higher that the desired design humidity". I hope this helps.