You can always pick up someone for RBI's or for AVG, but (and I think this is the same thing that New Zealand Fan is saying) most guys that hit for AVG that are available will be bad for your other categories. That said, there are always some suprises that you can get that hit for AVG and don't hurt you in other areas. Last year, guys like Mark Loretta, Frank Catalanotto, Shannon Stewart were pretty much available anytime you needed them to plug a hole left by injury without hurting your average.
As far as SB's go, depending on what kind of league you're in, there are no cheap SB's. Even Dave Roberts will go in the draft. Trying to cover SB's with waiver pickups is iffy at best. Somebody may have a breakout year, and if you pull the trigger fastest, you can grab them, but if you punt SB's during the draft, you're pretty much locked into trading for SB's, or punting them for the whole season.
For pitchers, again, it depends on the kind of league you're in. If it's a yahoo league, I'm not sure what it is, but there's something about the settings in a standard public league that makes it adantageous to have 3 or 4 good closers. Mostly because of the inning limit, and the fact that the best closers are 4 category players, just like the best starters.
K's usually take care of themselves if you draft guys for W or SV. There are some prototypical KO pitchers like Wood or Vazquez, who you can target to take care of the category, but usually you don't have to worry about it.