It very much depends on the sector. Early morning arrival before the LHR rush in good weather, Flight Plan Fuel is fine. Same applies to most multi runway environments.
So if that applies, then happy commiting down to 30 mins fuel planned on landing plus say 15 minutes, so say 7.5 tonnes on a 747 - thus allowing for a G/A and second approach.
In general, a multi runway operation (or v close alternate) allows an greater degree of flexibility.
A single runway airport would give greater reason to add fuel, as of course would be a remote airport (Perth, Bermuda, etc) and adverse weather.
Any large scale CB activity/snow down the eastern seaboard, for example, would see me desiring at least an hours extra fuel off the Atlantic.