it depends on how often you use them, spinning up is more intensive then maintaining current velocity, but long periods of inactivity compensate for it...
But either way the changes should be too small to bother about.
Besides, ALL DRIVES FAIL eventually, so make sure you backup your data, and then it is not an issue (with 3 or 5 year warranty, the drive is obsolete by the time the warranty is out, also the STANDARD for HDD is to NOT require proof of purchase to get a replacement, just send it in.)