Untreated, soft juvenile wood in window frames and trim will rot quickly if it’s in frequent contact with puddled rainwater—especially if the area can’t readily drain or dry out. End grain is particularly prone to soaking up water. And in hot, humid climates, the risk of rot increases.
It only takes a little air space to allow the base of a window or door unit to drain and dry when it gets wet. Use plastic shims to hold the foot of the unit up from the waterproofed sill framing. And cut brick mold or trim just a tad short to leave a drainable space at the bottom.