In SX Microcontrollers, SX/B Compiler and SX-Key Tool, George Herzog wrote:

The RC circuit is about 7%, older resonators about 3%, newer resontors down to 1%, and crystals even lower.
Resonators are usually good enough for UART.

Frankly, timekeeping more demanding than the UART and it is far better to be connected to a network that corrects the time automatically if the issue is important.  Temperature controlled crystals [TXO] are expensive and require maintance to remain accurate.  Also, extremely accurate timekeeping seems to be military or scientific hardware and begins to raise questions of justifying why you want such.  It become a big headache both in cost and justification.  If in doubt, you can buy timekeeping chips with backup battery that are more accurate than you can build.  I believe Maxim has some.

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