There's a lot been said over the years about this!. The reason usually quoted for wanting to do this is that CMOS inputs can 'drift about' if not connected (as they have a high input impedance) - now if they are high or low, that's fine - but if they are drifting around the middle it can increase the current consumption of the circuit, this increase could be considerable as PIC's are pretty low current devices.
So, you can tie them high or low - preferably with resistors, but bearing in mind that tying them high will feed some current into the inputs. Probably the best idea is to try them either way and measure the current consumption.
Or you could simply set them as output pins!. Again, try measuring the current and see which is best.