It's a tough call, for sure. When you're bootstrapping cash is tight! I think it might depend on your background and comfort level with financial management. If you're already financially savvy maybe you can push through a bit longer. As one page said about
fractional cfo services a reason that some wait is to avoid unnecessary overhead.
I've seen companies go both ways. Some bootstrapers swear that having a fractional CFO early on saved then from disasters related to cash flow etc. Others say it's an unnecessary expense until you get past a certain revenue point.