Baseball America says so.
The guys over at BA broke down how each team acquired their top 30 prospects and in the notes that the Reds had 2 Rule V guys listed, Jared Burton (how he didn't go over the roster time, I don't know) and Sergio Valenzuela. Its interesting to note the breakdown of the Reds prospects though:
Here is where the Reds rank among the 30 teams in each categoryCode:ORG COLL JC HS D/F NDFA INTL TRADE RULE 5 INDY FA Reds 15 0 7 0 0 3 3 2 0 0
College - 6th (tie)
JC - 22nd (tie)
HS - 13th (tie)
D/F - 20th (tie)
NDFA - the most any team had was 1
International - 19th (tie)
Trade - 9th (tie)
Rule V - 1st
Indy - only three teams even had 1.
Free agents - only the Pirates had more than 1