Screwing the bar down will make it more stable so the guitar stays in tune better. Sustain depends more on the bridge than the stopbar, the more secure the bridge the better the sustain. (I'm sure there is some physics principle to explain this but I don't know what it is, maybe it has something to do with vibration transfer or something.) Also I read someplace that the string angle across the bridge should be about 15 degrees, I guess thats the minimum needed to avoid vibration or slipage. One last thing to consider is the saddles, are they round or wedge shaped? Wedge shaped saddles create a sharper angle and may increase your tendacy to break strings if the angle of the strings across the bridge is too great, (any time you put a bend in a string it creates a weak point.) If you want to raise the bar and keep it stable, a few washers under each side will do the trick, this works under the bridge too, you just have to be more exact in the thickness of the washers.