I must say I use both. First person is great for emphasis, my supervisors are happy with this, and certainly from an ontological standpoint I believe 1st person is appropriate. I struggled at first though, esp. as I'm doing a mixed methods PhD so have both qual and quant in it, so being consistent with the language can be hard. I guess it's important to have your supervisors on side, and maybe think about who is likely to examine your thesis..?