I agree Zip ties will not give uniform pressure, Right at the head the plastic will always have a small 90 degree bend, That bend will be looser than the rest of the zip tie.

Now I've used these [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXET33&P=SM] Little guys[/link] for a little piece of mind, but I've also has systems not leak for years with out them.

good luck