following this logic i would suggest to do everything onMouseOver
So let us think of this. This of course needs to be considered per-situation basis:
Buttons that do one function only may be onMouseDown or -Up, doesn’t really matter, so best to keep them onMouseUp for people with sensitive mouse-finger.
Buttons that have permanent effect on song data should be equipped with hesitate-option and execution kept on onMouseUp. Especially save and trash buttons are excellent examples of these.
Buttons that have or could have extended functionality and don’t especially need hesitate-option should be stretched beyond convention, but logical behaviour should be preserved of course.
Menus can really improve from altered functionality, like you all are familiar in windows. That would be an improvement.
Also panel view buttons could stretch their functionality serving like menu bars opening up views instead of menu lists. Coupled with keyboard focus unlock this could also fasten focus selection if panels are to have multiple focus areas by dragging directly into desired area and then releasing. (Second thought strikethrough, questionable statement. Feel free to think of it.)
I think this is what topic starter was getting at in the first place - GUI controlling and especially menu selections could benefit from onMouseDown functions. The only questions that remains is if people are willing to adapt and if there is any adaptation needed at all, given that very many Windows-menus already open on mouseDown. To know we would need user input, and I, as an user, would see this as an improvement.
The real problem lies in that like many other threads this discussion is not executed onTopic.
I rely on the dragout behavior Byte-Smasher is talking about in all kinds of applications. Changing this for a very marginal, perceived increase in UI responsiveness seems like a terrible tradeoff to me.
If we’re going to break common, fundamental UI conventions lets try to get way more in exchange than this.
Besides, it’s a tracker, why spend time on fine-tuning mouse interactions?
\m/ >.< \m/
Hazaa 
To the topic starter: Check out the keyboard shortcuts
Try comparing Opera’s, Firefox’s or Explorer’s tabs with Google Chrome’s. It might seem like a small amount of time, but Chrome feels much nicer for whatever reason.
Yup. All if poss…
Noooooooooooooooo 
Actually, the mouse is needed really to turn on/off tracks and subtracks, especially if you’re editing in another track at that moment. For this reason, I’d like those buttons to be bigger as well.
Here’s food for thought: All keyboard shortcuts actually operate on the mouse down principle - yay! 
Keyboard is king!
I like the idea, as an option (default, off).
I’d have it on for live sets which need the tight timing, and off for regular use.
Of course, every extra preference adds clutter and reduces the commonality between one user’s experience and the next…
Not if it goes in an ‘advanced’ section, which can be put aside from the others.
Yes, though that would apply to things more like different keyboard shortcuts I’m guessing.