I park where there is a space unless it's a double yellow, permit holders only, disabled or parents with kid's bay.
I usually reverse into a bay and try make sure that I'm relatively straight and central, which is usually dependant on how well the cars in the bays next to are parked, if some idiot has parked right on the line or at quite an angle then with the best will in the world you can't put your car in the bay in the text book way.
And if the offending car next to you leaves first it leaves you looking like the sloppy parker!
I'd never park in a disabled bay, nor a parents bay. Although I have to say I do disagree with the need for a parents bay. Plenty of people walk into towns with pushchairs and children, why some people need to be a couple of metres closer to a supermarket door is beyond me.
Elsewhere I really hate people who park in stupid places on the road, too close to the mouth of a junction or on a corner for example.