Yo! New to MMOs, have a few questions!

[Frizze]

Having a ton of fun! I finally got into one of the Classes that required a second job to advance.

Question: Would it be recommended to grind in all of the job classes to lvl 15, 30 for any reason? Would I need to do so for any MSQ?

If you do crafting, most people recommend getting everything to 15(or higher) for cross class skills. Its not an issue at all for gathering classes. Combat classes really only benefit each other via the cross-class skills and/or shared gear. Some classes have more important cross class skills then others though, so this isnt really spread evenly. So Luna has the right of it, figure out which class(s) you find most interesting and either ask or look up the skills they need most. That will give you a decent guide to where your leveling interest can best be applied. All you get for doing every class is an extra title or two(and those require level 50 or 60 each).

And finally, nothing in the MSQ will require anything more then one class at an appropriate level. The story accommodates any combat class/job for each step, so you can use what you like.