I think it's very situational. If I am rebuilding, I couldn't care less about those 20 games. To be fair though I think I am better at rebuilding the Mike way than I used to be. The fuzzier ratings make the 10th pick about as good as the #3 pick, so I grab some ok guys at the end of FA where I didn't always do that in the past.
if I'm competing for a title, they are up day 1. More often than not I leave them down 20 days as I have a decent replacement from late in FA, but it's not the automatic 20 days no matter what that it used to be. The Cubs don't feel bad about leaving Bryant down. The Sox are fine leaving moncada down. It's certainly the right long-term call, but not always the right short term call.
Position matters some too. I'm more likely to call up a rp than a stud sp. I've had mixed results with young players developing at the ml level.