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I keep a builder free to upgrade walls, as I usually play often and get lots of gold. Since final upgrades for TH 8 take a week or so, having 8 million gold lying around isn't very good. I personally spend all my gold on walls before I go offline for a while, so I know nobody will steal it, and while it goes to good use. My walls are almost all level 7 going on 8, so I need to use the money somehow. You can always just save money before a builder ends so you can upgrade another defense.
I have lost about 150k gold to raids (good ones too) this past week. Each raid gets about 7k gold, so I never get pwned by high THs or anything.
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It does depend. I personally have had a builder open 99% of the time I was awake while going through town hall 7, it's very helpful in getting rid of spare gold especially when you are trying to save up for other upgrades with elixir or dark elixir since having too much of several resources will just make you an even bigger target. One good idea is to start looking at overlaps in your upgrade times so you can ideally place your one free builder on an upgrade just before you go to bed, and in the morning or sometime in the next day another builder will free up to be your wall guy. hope this helps!
