Any "rule of thumb" will depend on the municipality and the state of the real estate market. For instance, a few years ago in the Edmonton market, properties in good condition were selling for around 10 - 15% above tax-assessed value. This is by no means accurate and is subject to change without notice, but can be helpful information nonetheless. Your realtor may know what the current rule of thumb is for your area.
As for accuracy, the property that I bought at that time had been incorrectly assessed at $394k. I figured it should have been assessed for around $310k and we paid $335k for the property. When we got the new assessment a few months later, it was for $315k.