Should I use an external voltage reference with the PIC24F16KL402?
The main time to use an external reference will be when the in-built band gap reference doesn't offer enough accuracy for your application. I couldn't find a mention of 1% accuracy although it wouldn't surprise me if that was the typical accuracy. However from the relevant part of the table on page 201 of the PIC24F16KL402 Datasheet:



So over the full operating range the reference may be as far as 5% from the typical voltage. From a comment you seem to have a bit of a misconception that an external voltage reference relies on a stable supply voltage, but the very idea of them is to provide a stable output regardless of the supply voltage. The main thing to be careful of is to provide adequate filtering on the reference. Here's one recommended method from the LM4040 voltage reference datasheet:



While the internal band gap reference may be accurate enough for your application another option to play it safe may be to design your PCB with footprints for the reference and leave it unpopulated if it ends up not being required. The above reference is in a SOT-23 package and you can easily find the three passives in a 0603 package.