I have Webb ranked #29 and Peterson at #42 in the draft guide. Those rankings assume "normal" scoring systems without points for reward yardage. Peterson is a top 5 DB in the majority of leagues that reward for return yards/TDs and Webb is a top 10-15, so I definitely think it pays to wait on those guys if you can steal them later in drafts.
Here is an article I recently wrote that covers off on this topic:
http://www.idpguru.com/2012/08/2012-idp-return-men.html*Please note, D. Manning may not end up returning kicks for HOU this season.
Let me know if you have any other questions.
Guru