Gonzo, usually the flight level that we have filed, as far as I can tell, is the optimum flight level anyway. There may be the odd occasion when it isn't. So personally I don't always try and get a higher level than that filed because I don't always need to (going as high as possible isn't necessarily more efficient).
The bit wot annoys me is when we can't ever get to our filed flight level. As you point out this may be because someone else has nicked it when they shouldn't have. This can become a problem, more so lately, because we might not be carrying enough fuel for one or two levels lower than we expected.
This situation is where I expect flexibility. NOT where people are asking for higher levels unnecessarily.
On the 'Directs' point, I don't bother asking.