Considering how cheap these are, they're truly spectacularly accurate [to within a few tens of parts-per-million, worst-case!]
If you're worried about loading of the circuit you connect them to, a simple JFET source-follower
http://www.nutsvolts.com/uploads/wyg...rston_Fig6.jpg
will give you a seriously-high input impedance - good-enough that a few inches of insulated wire stuffed into the signal-frequency side of your existing oscillator will 'sniff' enough RF to give a good count.