If op-amp offset trims are provided, but not needed, what is the best practise for connecting them?
If nothing is said about it in the data sheet and application notes (as does occur sometimes), leave them floating. 

If there is direction in the data sheet or application notes, follow it precisely, as @IC_designer_Rimpelbekkie says. I can't recall any exceptions, but that doesn't mean that there are not one or two types of op-amps somewhere that require you to do something different. 

In any case, keep the traces (if any) to the offset null pins short and symmetrical- the offset pins are generally connected directly to the long tailed pair and have the potential to pick up noise. 

A general comment inspired by the terminology you used - it is not a good idea to use the offset null adjustment for "trimming" offset to a non-zero value. Any adjustment so made will tend to degrade the temperature coefficient of the amplifier (usually something like PTAT (proportional to absolute temperature)). If you need (say) a 2mV offset it's usually best to create it with a stable reference voltage rather than deliberately unbalancing a balanced circuit.