If they are, expect to reimburse expenses. Many volunteers work on several projects at once and it can be hard for them to guess to which you are referring. Failure to do so could result in your request being overlooked. Follow this link to see some examples of good and bad requests. Please bear in mind that many of the sources used for these lookups are the result of the dedication and hard work of a family history society.
Many societies produce publications related to their county.
You may want to consider purchasing your own copy of relevant resources to aid your research. You may also wish to consider supporting research in this area by joining the society. Details can be found at their respective web-sites or see Genuki's Family History Society listings . 1851 Cornwall Census Surname Index, Vol. Joan has compiled a list of those resident in Devon at the 1851 census, but who were Cornish-born.