So when I buy postage via pbs, how seriously does the post office/pbs take the "mail date"? 

I just realized that I hardly ever change the box that says "Mail Date."  It's next to the wrapped weight/book weight menu.  I think it's always today's date.  Whoops. 

I think as long as you are mailing it within a few days you should be fine.  I don't think they look that close but I could be wrong

I think it depends on your local post office, and in my experience in my town, there tends to be more of a problem if it over the 13 oz.  When I was trading DVDs, I always printed postage and mailed whenever and never had a problem until I did it with a package that was a little over a pound that got returned.  I asked the person in the post office and she said the person picking it up is more likely to look at the date if it feels heavy since they have to make sure it has the correct postage to be placed in the box. Now if I forget to change the date and need to mail after the next morning from the date on the package I just go to the post office and tell the clerk and he stamps it in for me.

when I used the PBS postage you can always change the date of mailing before you print the postage

like If I was on the site say today (it being a holiday) I would have it print either tomorrow or Saturdays date instead

and yes you can mail it earlier then the date printed (i like if I said Saturday but mailed it Friday) it will be acceptable;

it's supposed to be same day and that's why you will see a 0.00 meter strip/ inprint on the back of some business letters. The problem is we (as in the Post Office) have service standards and scores to meet. So when you do things like a lady did yesterday where she dropped off about 20 packages of Click-n-Ship priority mail with delivery confirmation and all of her mail dates were around 18 November on the packages then guess who gets blamed for not delivering them in a timely manner? Which is why we will print a 0.00 PVI label or use a round dater on them.

Kevin