Worst rule of thumb I ever saw!
Sorry.
Upgrading TH early - Cons...
- Loot penalty - this sucks.
- Because of loot penalty - it is harder to save for upgrades, of which, you have more to do.
- THs below you can now take more loot from you (thus making it harder for you to keep your loot).
- Cost of searching for battles costs more loot.
- Because of loot penalty you need to raise trophies to hit bases with decent loot available - higher trophies - tougher bases - tougher attacks on your premmie base (thus making it harder for you to keep your loot).
Upgrading TH early - Pros...
- Lots of shiny new low level defences (that you can't afford to upgrade).
- More walls (that you can't afford to upgrade).
- Better troops upgrades (that you can't afford to upgrade).
I know that some people skip the last few lab bits or walls so that builders don't stand idle but I'm talking about upgrading waaay to early.
Personally I've regretted every upgrade that I decided not to do if I could of done it at the town hall below (BK & lab upgrades). Its almost always easier to do it if you can at the lowest TH level possible.
tldr - you make your own life harder by upgrading your town hall too early.