In terms of ranking for the draft, keep in mind what your scouting budget is. The further you are from 20 for either college or HS, the further off your scouts are going to be. I've noticed that scouts will over-project vL and vR, and pitch quality, by quite a bit if your scouting budget is 15 or less.
Rank as many players as you can. Put crap pitchers all the way at the bottom, and then thoroughly rank your Top 100. As others have suggested, don't avoid excellent hitters in your rankings.
I tend to draft a lot of pitchers, and so my Top 50 might include 35-40 pitchers. Figure that if you rank really thoroughly, you'll land a very good SP in the first round (a front-end SP if you are drafting in the top 12-15 overall). Supplemental 1st round and 2nd round might land you a back-end SP or above-average RP. You might get lucky and find a late-inning RP in those rounds. 3rd-5th rounds you may still be able to grab a very nice RP.
Remember that guys with lower DU have lower OVRs, and some owners don't rank very deep into the draft. So the guy you rank 30th may still be around in the 4th or 5th round. Also, remember that guys may have solid ratings in most categories but vL, and be very useful major-leaguers as RHSs -- capable of giving you 50 IP in that role. Such a guy might project as, say, 75 CN, 42/63 splits, and pitches of 85-70-55. Even if the guy has limited ST/DU, he can be very useful as an RHS, and be someone you can grab in the 4th-6th rounds.
LHS, by comparison, has limited usefulness at the MLB level. A lot of owners have tried to use a guy here, and wound up with a 13th pitcher who gives them 15 IP in the whole season.