We do our Dungeons a few ways.
Most runs are first come first serve. We tend to let new members join a run over someone who has done it 30 times. Unless of course we need a experienced mage or tank to ensure survival or lead the group through. We're not afraid to wipe either.

Scheduled: Dedicated times based on the time available around the LS. Rotates in/out with NM hunts based on member needs.
Pick-up Linkshell Groups: If a few R50 people seem interested, we'll put together a group on a whim.

Dungeon Run Types
Efficiency, vs Experimentation. We don't like to lock members into playing dedicated roles, archer builds etc. We do run known successful builds though too, depending on if we have a mid-core or a hard-core group.
Experience Run: We try to get less experienced members through the dungeon and practice. Usually includes Task Oriented type.
Task Oriented: Completion of specific goals, such as Grand Company Quest completion.

Sure we could just try and go for efficiency every time, but I know our members would lose interest. We're in this game to have fun right? We try to show everyone you don't have to be "the best" to enjoy the dungeon. Efficiency has its place, and that is why we doth both various styles of runs.