I asked my advisor when I should put my thesis on arXiv and he said I should do it after my defense, but not earlier. (And I complied.) It seems to be a standard practice (at least in theoretical physics).
If there is demand, (very likely) you can mail your thesis privately (I did a few times). But putting it publicly may give some risk:
- it could look as if you have already defended it,
- in the case of failing (which, depends on country and procedures, may be possible or unlikely) it is kind of awkward.
Plus, the standard caveats: ask your advisor, check your institution policy etc.