There are free map sites everywhere, and there are also free gpx files with the MMWT and MR loaded so that you literally have step by step directions. They're not really necessary, but if it helps you, then I say do it.
Having used both my iPhone and my Garmin Oregon on many a trail now, I can honestly say that while the iPhone is convenient and nice, it's just not as good a GPS as an actual dedicated GPS unit. They don't track as well, the battery life is always suspect, and they're usually not as durable for hiking and climbing.
I like having my iPhone with me, and carry it with a Bad Elf for just in case purposes, but it does not replace my Oregon for GPS purposes.
Here's a site with the gpx files for the MMWT and MR, as well as quite a few others in the area:
Whitney Trail Tracks and More 
