Always quote them with "$var" unless you are either absolutely sure they contain no spaces or you actually want to expand spaces. In the former case, you may want to use "$var" nevertheless for consistency.

Also: ${var} is more readable than $var, especially in cases like "$destdir$prefix/$file". "${destdir}${prefix}/${file}" is a little bit easier to digest. I'm not really sure if I want to use the ${var} form for all variables, though. With "${var}", it might get a little redundant. OTOH, in cases where you don't need quotation (like zsh scripts), ${var} looks better.

Unrelated, but maybe also interesting: I'm currently editing without syntax highlighting. As I can no longer rely on it to help me find stuff, it forces me to write a little more readable code. So I guess I'll keep it disabled.