Regarding choosing focal lengths, I choose a maximum of 2 (typically just 1) depending on my mood and the anticipated subject.
Sometimes I'm in the mood for getting really close to my subjects for instance, but sometimes I don't think my subjects will react well to that proximity. In this case I'd choose a moderate wide, in my case a 35mm. Different subjects and mood will lead me to either a 21 or 35 as my main kit. On the other hand, when travelling light I'll just take 1 lens - I took just a 21mm to Long Island for a week, and the next fortnight took only a 35mm to Italy. In both cases I shot what worked for what I had - my mood was somewhat governed by the lens I had on I suppose.
The exception for me is longer focal lengths - I'd never go out with only a 90mm for example. For me, these are subject and circumstance driven lenses, and I only use them in a secondary way when I can't get close enough for the framing I want. I rarely use them to compress perspective which I find interesting as I really like to expand perspective with wides. I also rarely use a longer focal length unless it has its own body - I'm not likely to change to it in the field so to speak. This makes me think that I'm either not comfortable or don't know how to use longer focal lengths.