Writing ‘Pretty’ Permalinks

I have read the codex entries about writing permalinks and have a couple of questions.

a) The codex says “make sure to end a virtual site structure with either %post_id% or %postname%”

I want to end with %post_id% so can I end my permalink with /postid=%post_id/ in order to make it look more readable or does it need to be plain /%post_id%/
(i.e. is it ok to put ordinary text between the ‘/’ and the ‘%’)

b) Should I terminate my permalink with / or not?
(i.e. should it end %post_id%/ or just %post_id%)

c)The Codex says “/%year%/%monthnum%/%day%/%postname%/ is always a good start”. However, based on the advice about writing ‘pretty permalinks’ and SEO it would seem to make more sense to have %postname% first and then the year/month/date. Is it ok to do it this way or might it cause any problems?

c) Does anyone think there are any real advantages to adding ‘category’ at the begining of a permalink in order to optimise ranking in the search engines, as has been sugested? I don’t know enough about how search engines work to know the answer and would like some feedback.

PS I’m on Wp 1.5.2

Thanks