With a new web site, does anyone know if I need to wait for a PR update before I start placing well for my title tags / keywords? Thanks.
PR is always updating and doesn't necessarily reflect it's current value on the toolbar. Actual PR could be higher or lower than the toolbar says as it's just a reporting feature that gets updated whenever Google gets to it. Outside that, there is no PR requirement to rank well for phrases your site is optimized for it's just another factor that can help though is debated. So, your answer is "no" if you mean the toolbar PR, and "probably not" if you mean true PR (which none of us know)
Gaining your PR is not a matter of prediction... otherwise It's a matter of quality links that you get.
If you are not talking about highly competitive keywords, you can probably see some decent rankings fairly quickly, even with low PR, if you accomplish the various aspects of good SEO. I launch new pages and see many of them begin to score well in the SERPs even before getting any toolbar PR.
You don't need PR to rank for low competition keyword. Let's say you are optimizing for phrase "noone optimizes for this one", you can get ranked with a brand new website very quickly. On the other hand, if you have mild competition, you can still rank for those without having a toolbar PR. You will see your PR only after the toolbar PR is updated, but google is already using a 'real PR' while calculating your rank.