As emphasized in the articles Piano Values and Antique Piano Values, a piano's fair market value may not reflect its true quality as a musical instrument, and the older a piano is the more this rule generally holds. Antique piano appraisal might thus help to determine a reasonable market price for a piano, but it might tell you little about how good (or bad) the family heirloom actually is.
Since piano prices fluctuate with supply and demand, piano appraisal is only relevant when buying or selling a piano. Any appraisal can only assess a current fair market price for a piano, not its intrinsic value. It's therefore important for the appraisal of your piano to be up-to-date.
With antique pianos, the method of determining a fair price for a piano by looking up its historical selling data in a blue book of pianos, as described in the article Piano Appraisal, is of limited use: the Blue Book contains data going back fifty or so years, and an antique piano will be older still.
Antique pianos are even more sensitive to the factors that account for piano value, and fair market prices for two pianos of the same model may well differ wildly. Sometimes the modest cost of appraisal is greater than the appraised value!
The vast majority of antique pianos tend to be in poor condition, although some of them can be repaired or even rebuilt. (Although it is often worth the money, the cost of rebuilding a piano can eclipse that of a similar used piano in better condition.) A cracked soundboard as a result of careless placement next to a heater, or highly fluctuating humidity, will permanently affect a piano's value. Worn hammers and rusty strings require replacement, and the action will need to be regulated in order to restore it to good playing condition.
Because of these factors, antique piano appraisal needs to be performed by a professional piano technician, who can recommend any necessary maintenance or repairs as well as offer guidance to both buyers and sellers.