In reply to Smelly Fox:
When living and climbing in the Lakes for two years between 2012 and 2014, I have to say I found FRCC's +/- system helped with route choice. If I was having a especially good day, we'd go for a + in the grade boundary, if it felt like a wobbler was in the pipeline, drop it a bit and go for a - grade.
It also feels like it helps more with the longer routes. Routes with sustained difficulties tend to be at the + end of the spectrum. One hard pitch but then more amenable pitches otherwise would give a -.
I can see that an argument is there for just dropping a grade if you feel like you're on an off day, but sometimes just a small increment down is enough, and is actually more enjoyable.
Either way, it's each to their own, and I appreciate the extra detail the FRCC put into their guides.
Disclaimer: I don't always agree with their assignment of +/-, but it's usually pretty accurate.