That is correct, you may use manufacturer's direction to establish frequency. When the manufacturer states no frequency, monthly still applies.
Here is the link and the actual entry of approval from the FDA regarding the calibration of thermometers:
http://www.fda.gov/cber/blood/exceptions.htm
21 CFR 606.60(: Calibrate digital thermometer according to the schedule recommended by manufacturer, instead of monthly as required by regulation. [Variance request no longer needed when following manufacturer’s instructions, as required by 21 CFR 606.65(e)] Calibrate Traceable Memory Monitoring Thermometer quarterly, instead of monthly as required by regulation. The thermometer’s package insert did not include recommendations for frequency of calibration; therefore approval was based on review of submitted data.