That's this month's question, and I think the 'across-the-board', unanimous answer is Nooooooo!
And I think it's mostly true for me, too. When painting miniatures, especially rank and file models for 40k, there are plenty of hidden areas. Sometimes I might paint a detail before I decide on the accessory to attach - for instance, the Space Marine backpacks, which have grilles and other markings in the space between the figure's back and his pack.
Is there any point in painting the detail? Not really- it's a dark, obscured area, but it could be good practice for new techniques, or other markings you would place elsewhere. If no one can see it, it's yours for the taking.
Other times, for instance when I'm building a diorama, and haven't planned it (60% of the time, every time), I'll detail something only to stick foliage around it.
But usually No. Life's too short.