Do you put plumbers putty on rain downspouts, or roofing tiles? Plumbing traps are designed to permit water to flow past...but whether they're sealed during a clog you'll have to consider. Drains don't clog much...and when they do there's not often much static head of water weight - provided all orings or gaskets are in good condition that's all you should need...but I cannot see your trap/pipes - you say pvc cement so they're white/grey plastic...or possibly abs...you should absolutely NOT cement them...as that defeats the purpose of being able to easily disassemble. The rubber gaskets should maintain the seal provided your alignments are good. The goal is to get the gaskets to do the proper sealing.