Crystal load capacitance matters in two regards:
- the required load capacitance to achieve the nominal frequency, as specified in the crystal datasheet. The specification is sometimes misunderstood, it's the total capacitance seen by the crystal, not the value of the capacitors connected to both crystal terminals.
- the capacitance range that allows reliable operation of the crystal oscillator. Although it depends weakly on the crystal series resistance, it's mostly given by the oscillator parameters and is specified in the microcontroller datasheet.
In terms of reliable operation over production process, voltage and temperature variation, you'll look primarily for the second specification.
Exact frequency matters for applications like RTC or RF transmitters.