Regardless of what you do, don't just drop in "high temperature grease" (I'm inferring you're thinking of dropping in just some ole ordinary chassis grease or multipurpose grease). Different greases have different purposes (my airplane, for example, uses three different kinds), and high-temp bearing/chassis grease is NOT appropriate for CV joints. You'll want to use the proper molybdenum grease, designed for CV joints.
Also, make sure to use the proper amount. A lot of folks tend to think that if a little is good, a lot is better; not so. Too much will cause too much expansion and can blow out the boot.
To your question directly: I suggest the answer to that depends on the source for the shaft assembly. If you buy OEM, or buy a good rebuild from a reputable company (e.g., Empi, OEM supplier, or a RAXLES rebuild) then you're fine bolting on and going. But, if you're buying a potentially cheaper-made one from, say, Pep Boys, then I suggest you're better off having your guy pull them apart and repacking.
As an aside, it's always a good idea with a CV-joint-equipped car to carry with you a handful of CV joint bands, the right grease, some really good duct tape, and the appropriate band tool. Loosing a band or a boot can ruin a weekend, and quick-fixes can get you through Sunday.
Greg