If you plan on running instances always have a DE'er.

If you aren't terribly concerned about leveling speed then pick whatever's best for your toons (Tailoring for cloth casters, etc.)

I run with my main toon having mining/skinning so I can pickup those nodes whenever I see them(One of my alts is a decked out JC'er so having some adamantite is nice), and supply leatherworkers with stuff I kill is just an extra click.