The Oxford Companion to the English Language page 75:
There was formerly a respectable tradition (17-19c) of using the apostrophe for noun plurals, especially in loanwords ending in a vowel (as in We do confess Errata's, Leonard Lichfield, 1641, and Comma's are used, Philip Luckcombe, 1771) and in the consonants s, z, ch, sh, (as in waltz's and cotillions, Washington Irving, 1804). Although this practice is rare in 20c standard usage, the apostrophe of plurality continues in at least five areas: (1) with abbreviations such as V.I.P.'s or VIP's, although such forms as VIPs are now widespread. (2) With letters of the alphabet, as in His i's are just like his a's and Dot your i's and cross your t's. In the phrase do's and don'ts, the apostrophe of plurality occurs in the first word but not the second, which has the apostrophe of omission: by and large, the use of two apostrophes close together (as in don't's) is avoided. (3) In decade dates, such as the 1980's, although such apostrophe-free forms as the 1980s are widespread, as are such truncations as the '80s, the form the '80's being unlikely. (4) In family names, especially if they end in -s, as in keeping up with the Jones's, as opposed to the Joneses, a form that is also common. (5) in the non-standard ('illiterate') use often called in BrE the greengrocer's apostrophe, as in apple's 55p per lb and We sell the original shepherds pie's (notice in a shop window, Canterbury, England).