I guess it depend of the accuracy you need. If you use the internal Vref, when the PSU voltage swing, your accuracy will swing with accordingly right?
Yes absolutely, but if you have 20mV noice or swing on the supply rail with a sensor output of 10mV/° you have 2° worth of noice. If you boost the signal to 100mV/° the same noice on the supply rail is now "worth" 0.2° instead.

PICante,
In a previous reply I sugested a Vref of 1.024V but after reading the 16F88 datasheet that may not have been a very good advice. Vref needs to be 2V minimum according to the datasheet so keep that in mind if you go for an external reference.

/Henrik.