YES, if you live in an area that experiences freeze-thaw conditions, the expansion joint should be sealed to keep freezing water from destroying your coping and tile. Unfortunately, most pool builders do not caulk their pools even though they know better. You should wait 20-30 days for the new concrete decking to cure before any type of sealant is applied. Use a polyurethane caulk, the self-leveling is the best.