quite often! i have many quilts that have both hand and machine quilting on them- and i've had 'customer quilts' come in that were hand quilted & the owner now wants machine quilting added to them. if it works for your project- no reason not to do it that way- it's yours to do as you please.
and i've seen plenty of (show quilts) with both also- so don't think it (diminishes) anything.