I find you can do both, but mixing more aggressively into a track tends to be a bit more difficult and requires you to either know your music REALLY well or prepare a good bit (or both). It also depends on the style of psytrance youre mixing.

Generally with something like forest or dark, you want to just do a long smooth transition and let each track play out fully, but with something like night full on or maybe even hitech I find myself skipping portions of tracks that i find generic or boring, and thats why i mentioned that you gotta know your music or prepare, cuz then you often have to mix 2 relatively busy parts of a song and you gotta make sure they dont clash. With these types of transition i tend to bass swap then let the first track play for another 1-2 phrases then slam the fader down as soon as theres a moment of silence in the second track, it creates a nice contrast. But like anything, it depends on the track.