This was something I noticed over the weekend, for some reason, and have started thinking about - when you play a trill, do you start the trill with a hammer-on or a pull off? For example, if you're trilling on the D string between the open D and a 2nd fret E, do you start the trill by picking the open string or by picking the 2nd string?
I think historically I've been leading with, in this situation, an open string, which gives more of a slurred, behind the beat feel. I'm trying to get the hang of doing it both ways though, as the 2nd fret is a bit more on the beat and a little more aggressive, and feels easier to control for situations where you're going back and forth between trilled notes - say, the outro of SRV's cover of "Little Wing."
By 'hit-on' you mean hammer-on, right? In most cases I start with the lower, fretted note (or open, if the trill is between an open string and a fretted note). Basically, I start with the lower note. But , honestly, besides a bit of work to get them faster, I don't think I've ever really thought about it much.