Assuming you mean "in date fields on profiles" - from : Date Fields
"If you see a date from the 1600s or 1700s that has a slash in it, e.g. 1718/19, use the later of the two dates, e.g. 1719."
Full double-dating can then be used in Biography section.
EDIT - though the reason for selecting the second of the two years indicated in a double-date isn't indicated, that (perhaps among other reasons) permits direct comparison of dates. E.g., colonial records may indicate that a child was born April, 1701, and another child Feb, 1701. Those are both Julian dates, and, because March was the first month of the year in England and her colonies until (as Larry mentions) ~1752, the child born Feb, 1701, was born after April, 1701, some 10 months after April of the same (Julian calendar) year. In their profiles, the birthdates should be recorded as April, 1701, and February, 1702, clearly indicating (by comparison) the order of their birth. The biography for the latter could then indicate that the child was born "February, 1701/2" or "February, 1701/02."