Depends on how you play. If you have on problem with loot, go for it. However, if you aren't that active, stay. However, stay for a bit and upgrade some stuff. You don't need to upgrade stuff you probably won't need at th9(like valks). If you ask me, I am about a month into th9, and I rushed my th8 lab by 2 weeks. If I didn't do that, my account would be way less progresed than it is right now.
Btw, on galadon's F2P series, he rushed his lab like crazy.