I just built my first AR... Actually, I'm picking up my upper tomorrow (hopefully). I went ahead and did a .223/5.56 and plan to add a .22LR upper later. I started out thinking I'd buy a .22LR AR, but I went with the .223 because I felt the larger caliber might be the first to go in an AWB situation. Regardless, go ahead and get your stripped lower on-hand as that's the only "registered" part of the gun. After that, you can do as you please with it (short of an SBR or Fully Automatic).
You can always get multiple uppers later in different calibers (as long as they are compatible with the AR-15 Lower and not AR-10). I'm planning on adding a .22LR upper at some point soon. But you can get them via online retailers, etc. so I'd go ahead and get the lower before our illustrious leaders in Washington make any drastic moves.
Three old hunters were walking through the woods one day. The first looks over at the others and says, "Windy isn't it?" The second says, "I was sure it was Thursday." The third says, "Me too! The heck with hunting, let's go get a beer!"