Re: MCLR in PIC16F88
Whenever you do not use pins, you can either leave them as inputs and connect them to GND of Vdd, or you can make these pins outputs and so they do not need to be connected to anything.
Personally, I favour this second approach, since the pins are not connected to anything, so if I need one more pin to do something, then I just run a wire and use the pin, without having to disconnect anything.
Plus, many times during the debugging process I find it useful to toggle a bit at a certain poin in the code, so that I know when a certain event takes place, I can tell where the code is, so again, an unused and unconnected pin comes in handy.
Now to answer your questiom: when you make the /MCLR internal (connected to Vdd), the actual pin of the PIC becomes available as a general purpose I/O pin. This is nice, since in an 18-pin package, pins are scarce.
So, with a pin that is a general-purpose I/O now, you have to do one of the things I described above. Personally, i would make it an output and leave it unconnected.
But you cannot just leave it as an input and unconnected.