The accepted wisdom in fantasy baseball is that its impossible to predict relief wins and not worth the effort to try. There's some truth in that. You can look at all the factors, intelligently project Dan Wheeler to have a 6 wins, and a few bad breaks or lucky bounces over the course of the season can make the difference between him getting 9 wins or 3 wins. That said, in any type of gambling (and that's what fantasy baseball clearly is) you need to give yourself as much of an edge as possible. Ignoring relief wins entirely is passing up one of those edges. There are a number of factors that can help predict the win totals for relievers, and that's doubly true in a daily league where you can look for specific situations conducive to relief wins.
Pitcher Quality - This is an obvious one. A good reliever is more likely to win games than a bad one. However, if all you care about is wins, this is probably less important than you think.
Team Offense - The better the offense, the more likely the team will be able to come from behind and give their relievers wins.
Usage Patterns - How the manager uses his bullpen makes a BIG difference in reliever wins. Some managers use their 2 or 3 best relievers only in tie games or when they're ahead. Relievers on those teams are obviously going to get less wins than on a team where the manager won't hesitate to use his best relievers with the team down a run or two. Other teams (Oakland in recent years is one) will frequently bring in their closer in tie games in the 8th inning...reducing the potential for wins for their setup men.
Availability - This is the first factor that makes reliever wins easier to project in daily leagues than weekly leagues. If the reliever pitched yesterday, he's less likely to pitch today. Since most daily leagues allow lineup changes until right before gametime, you should always be able to bench relievers who pitched the previous day in favor of those who didn't.
Starter Quality - Relievers will generally get less use (and therefore less wins) on days when a good starting pitcher who is likely to pitch into the later innings starts.
Home vs. Away - This is one of the less obvious ones, but makes a fairly big difference for middle relievers. The following example shows why middle relievers will generally get more wins (and losses, but most leagues don't care about that) when they're at home. Game is tied after 5 innings. If the reliever is home, he comes in, pitches a scoreless 6th, and gets credited with the win when his team takes the lead for good in the bottom of the 6th. If the reliever is on the road, his team goes ahead in the top of the 6th (giving the starting pitcher the win), and the reliever only gets credited with a hold for his scoreless bottom of the 6th.