Maybe everyone knows this, I didn’t, mainly because I use connected clips, not secondaries, I don’t use audio dissolves much, and if I cut audio in a secondary the stock centered transition works fine most of the time. But, I discovered a little trick to have a nice A/B crossfade and use a transition to create the handles for you, and give you a “thumb” to move the cut point around.
Cut your audio in a secondary and stick a dissolve on the cut. The length doesn’t matter, it will just be a drag handle when you’re done – Then, just expand the audio and adjust/trim the A/B sides and fade handles as you normally would.
It’s really the same as if you just did it without the dissolve, except… you can now select the transition to adjust position of the expanded edit point using the <> + Shift (if needed) keys. Be aware, when it’s expanded, dragging the transition doesn’t move the cut, but when you collapse the audio, you can click and drag the transition and it moves he A/B handles/fades with it. Useful?
EDIT: The dissolve doesn’t do anything once you adjust the A/B handles, and you can pretty much do the same thing by selecting and dragging the edit point. What it does so is create some handles for you, and let you drag the edit point without changing tools. Saves a couple keystrokes and clicks… 🙂
Final Cut Pro X Babbling 
why would you want to use BOTH a cross-fade AND the handles. That makes no sense.
The dissolve transition doesn’t do anything once you change the a/b handles, it just allows you to adjust the transitions’ position in the storyline. which I guess you can do anyway by selecting and dragging the edit point. It’s really just a quick way to make A/B handles. Eliminates a couple keystrokes here and there…:-)
No, it actually ADDS some, since the transition plain isn’t needed. As you say yourself, if you do everything you do AFTER adding the transition but WITHOUT adding it, you can do the exact same thing with the trim tool. So adding the transition is superfluous.
Well, yes and no. It splits your handles for you, and lets you move the cut point without changing tools. Also it gives you a quick visual reminder that there’s a transition there when the components are collapsed collapsed. But yeah, nothing earth shattering. I just thought it was kinda cool.