IIRC, somewhere I heard it was a no-no because even on the same port, the switching time was uncertain and it could experience a transient moment where one pin is pulling high and another low.
I'm not certain whether that situation is always a problem or not, but it wouldn't really be a problem if it wasn't switching frequently anyways. It also wouldn't apply if you TRISTATED the pins to turn it off.
Did you check the spec sheet for the maximum current through Vss? And does 16F628A specify max current on a per-port basis? I've seen that on later PICs, dunno about this one.