Sure use that DSLR, why not?
I use an older Powershot as a light meter for 120. Not always (otherwise incident light meter), but sometimes. Works great. Just make sure to calibrate the ISO values of the P&S once. Earlier Canons are a bit conservative in their rating (64/80 ASA on the Canon equals 100 ASA for the film, something like that).
In any case, the histogram and the picture on the screen are great for getting a feel for a scene, especially difficult ones.
If I had an iPhone I would surely use the exposure meter app all the time. From what I've read it works wonderfully well.