I've seen it help for unranked applications. If you're currently a decently-ranked paid application, don't forget that in the transition back from free -> paid

I've seen it help for unranked applications. If you're currently a decently-ranked paid application, don't forget that in the transition back from free -> paid you will be unranked until you have sufficient paid purchases over time to rank again. You could end up throwing away whatever momentum you have :-)