the most important reason for the "pouring" is to reduce the inductance
of the upper side gorund plane.
most of the relevant field in the microstrip is confined in the volume
between the microstrip itself and the gorun plane beneath. there is a
phemomenon called fring effect, whereby fields generated on the upper
side of the microstrip come into play, but that occurs only at
frequencies of about 5GHZ and above, not at the usual frequencies we
talk about. it belong more to the microwaves' people realm.
leave it alone under 1GHz. you don't have to worry about that effect at
lower frequencies.
Saandy 4Z5KS