As a mid-range town hall 8, I bought my fifth builder about half way through th8. This is why I'm glad I did it and why I wish I had done it at town hall 7.
1. I firmly believe that the best way to spend gems is on builders or buying the King outright.
2. As a post mentioned earlier, always keep one builder handy for walls. This is especially true when you got 4 other upgrades going on that take 5 days or more.
3. As you approach the end of TH 7, you are planning the jump to TH 8. It will be SOOOO much faster for you to have 5 builders going on those new defenses that you are trying to rank up.
4. Right now I'm focusing on my air defense and tesla. Here's how I use all 5 of my builders effectively (everyone has their own way, mind you)
One builder for walls
One builder for camps, pumps, mines, storages, and barracks.
3 builders on defenses.
I start a defensive upgrade and, about 2 days before it finishes, I push for the gold on the next one. As you do this, elixir will flow, so keep your lab going and the second builder busy at all times. The you just keep going through the cycle. When all 3 defensive builders are busy with long upgrades like air defense, then start working hard on those walls. The builder for walls also serves to clear debris and gem boxes.
In short, I'm coming up on the end of th 8 and have multiple day builds. I still keep all 5 builders busy. Best investment I ever made. Not to mention that they can slow down troops for a second.
I really hope this, along with everyone else's great advice on here, will help you in what is indeed a tough call. If you have any other questions, please feel free to message me.
Best wishes,
Mud (clan Exodious)

