I have separate reels for SD and HD. But there's a good reason for my doing it this way - from 1994 to 2009 I worked exclusively in SD, since I was at a broadcast facility which was SD. After that, nearly everything I've worked on has been HD, so it was easy to base my separate reels on format.
I see documentary work all the time on PBS and other stations which mix SD and HD, windowing the SD stuff center screen. I've also seen some interesting approaches where the SD was in multiple windows, some with color or opacity effects on them. I would base my personal decision on the content - creativity is just fine on a demo reel.
I think it can also depend on the reel you're making. You can make a creative reel using SD footage on an HD reel. I love the way SD footage loses rez in an HD timeline - I deliberately blur it, oversharpen, mess with colors, zoom way in, etc. If you're worried about resolution, look at how you're presenting it. I've used several SD clips as tiny, layered fly-by clips in AE or Motion. You could also put 6 SD clips on a single HD frame and show 6 clips all at once.
I don't feel you have to lose your footage when the formats change, but you need to get creative about what you do.
I'm already seeing whispers of 4K TVs and how to integrate HD and 2K footage into a 4K timeline. Take it as a given that the formats will remain long enough for you to make your money and long enough for the manufacturers to make theirs and then they will all embrace a new format and we have to change everything all over again and then we'll be having this conversation again. ;)
I've just done what I assumed to be the norm. Up-rez the SD and add pillars. This then inter-cuts quite nicely with the HD material.
The best looking method I've found so far for the up-rez is to play out the material from one editing system into the other via SDI and then have the receiving machine (in this case an AJA Kona 3) perform the up-rez conversion.
For the pillars I've used Digital Juice JumpBacks with the 4x3 hole cut in After Effects. I've selected a few JumpBacks for their coloration, relative to the SD clips and for their simplicity. I've then de-saturated and darkened them so there's some motion going on, but nothing that will distract from the SD shows.