as said before the main requirement is going to be good access for the poor souls like me who have to come along and try to service and mend them, if we spend an hr dismantling your lovely cupboard to get at the boiler then its going on your bill!
we had one prat recently who had not only fully boxed in the boiler with only a small aperture left to use the control panel but shed also tiled all over the boxing, we had to dismantle the whole lot.
you will find that many of the boiler manufacturers state the minimum clearances required in the installation instructions.