From my understanding of Page Rank, and I could be wrong, posting your site on a low raked site won't lower your rank although it might raise the site you posted it on. If you post it on a site that has a higher one it should raise your site's rank provided Google counts that site in it's equations.
If you want a far more detailed explanation on how it works, take a look at this, but it's a very dry read.
Anyway, if you are advertising on as many pages as possible, I don't think you will lose PR, only gain it.
Also note that 1 link from a pagerank 7 page is like 100 links or more from a pagerank 6 page and so on down, a single link from a pagerank 6 page is like 100 links or more from a pagerank 5 page. So it takes literally millions of links from page 1 and 2 sites to equal a single link on a high page rank site. Hope this helps as well.
The answer is - it depends. Many people place too much importance on PR when in fact what you have to be asking yourself should be "is the website/page/section of a website I'm getting a link from relevant to my website". If the answer is yes - this will in no way affect negatively your "PR". If the answer is no - it is also possible that your PR would not be affected negatively. However, you might get punished if getting links from such places becomes a practice.
The bottom line - don't sweat too much over PR.
If your site was just starting, then you could get to PR 1-2 with links from just about anywhere - quantity is really what gets these PR levels. However, if you want to get in the PR 3-5 range, you'll need quality backlinks. Continually getting low quality links will not hurt you, but they will not prove significant in your ranking. I say continue to get links where you can and it will help - but if you want to have significant progress - you'll need to get links from higher quality sites.
The web is getting more and more saturated with poor quality links, which is lowering PR values, just like a neighborhood with declining home values. You'll need to cultivate the value of your current links by "linking to your links" - you can also increase your PR this way.
I would think of classified ads as a traffic source and thats it. Getting links from them is futile and ineffective. Google is all over that.