The "winter" is the best time to visit the Mariana Islands if you are not a great fan of high humidity.
The "dry" season will continue all the way through June. August is the peak time of stereotypical tropical summers.
See also Westin Resort Guam (21 Sep 03)
http://www.flyertalk.com/forum/showthread.php?t=174131
Any recent stay at Westin Hotel in Guam (23 Jan 02) (very cursory thread)
http://www.flyertalk.com/forum/showthread.php?t=169065