rmteo is spot-on.

I've used a ton of newer rev PICs' with internal osc specs @1% for serial communications, and never had a single glitch - as long as they were used in a controlled environment where the temparature remained failry constant.

Step outside this, where the temerature can change quickly, and you're just asking for trouble. In this case .. for sure go with a good quality external crystal, or even a quality resonator. Crystals beat resonators hands-down, and are a bit more expensive, but are worth the expense for peace of mind.

If you need the absolute best precision, then go with an external TTL oscillator. These beat all of the above, but they for sure will increase current demand on your power supply.

I have tested all-of-the-above in very harsh conditions here in Colorado, and I guarantee it does make a HUGE difference.