If anyone would like to look at the the original note in the Ifield monthly meeting register as mentioned in the research guide to the Quaker calendar, it can be seen in the Quaker dataset images at FMP, Ancestry and the Genealogist. The reference is RG6 1258 and the note is on image 222 at the latter two sites.
Interestingly, those who wrote entries in this particular register were at best inconsistent in adhering to this. As in other Quaker registers, many entries are not listed chronologically, which makes it difficult to determine what year was meant for March dates, but as early as 1675 some were given a double year. For example, a burial in Charlwood was given as '24th day of the 1st month 1675/6' (24 March 1675/6). In this example (which is not an isolated example), had the year been seen as starting on 1 March the double year would not have been necessary.
I think the answer here is to be cautious, and where events have not been entered in strict chronological order, a double year is not shown, and the date of the same event is not clarified in the Quarterly Meeting register, keep an open mind. With some registers, such as the Coggeshall MM register mentioned above, it may be possible to discover a 'normal practice' around the time concerned, but this won't always be possible.
Another avenue worth checking is the relevant parish register as some clergy recorded non-conformist births and burials which occurred in the parish.