I've worked on several mid-sized projects (including one 80KLOC Perl application suite) that used fairly minimal style guidelines, and a couple of projects that used coding standards.

What I've found works best is a combination of light-weight guidelines coupled with "educational" code reviews. Reviewing work by senior team members demonstrates what "good" is to junior team members. Reviewing work by junior team members is an opportunity for gentle correction. Even in the absence of standards or guidelines, senior people can usually form a concencus about where the boundaries around "good" are, and the rest of the team can learn by example.

This approach might not scale to a situation where all of the coders are peers, without an acknowledged senior/junior distinction.