Michael and Tom,
Really important question. Thanks for bringing it up.
Here is the current, terribly incomplete answer from http://www.wikitree.com/wiki/Date_Fields#Julian_vs._Gregorian_Calendar
WikiTree cannot accept dates such as 1753/54 so one year needs to be chosen.
Our general convention, as with names, is to use the standard that the person themselves would have known and used.
(Is this a sufficient answer? When the standard calendar changed from the Julian to Gregorian in 1752, was there a period of uncertainty?)
What do you think? Is there a cutoff date we can use?