There are at least 3 aspects to it I can see now, maybe there's more to it:
- you don't want to end up with less than 1095 days in case you forget a trip or not very accurate with dates, for example, you might think you had entered Canada before midnight, but it actually was recorded as after midnight and this comes up upon verification, stuff like that.
- put yourself in the shoes of an agent inspecting your passport during test/interview-- they should be positive you have >1095 days. If you have few or no travels and all stamps in a passport are there, then it's pretty straightforward and couple days buffer would be ok, but if you have lots of travels + missing stamps, for instance, cutting very close to 1095 may tick off a CIC agent into asking for more evidence, since they can use discretion in addition to a set of explicit rules when to issue request for additional info. It's their responsibility to confirm your residence on balance of documents and story provided.
- periods you'd have trouble accounting for in case of RQ/judge hearing-- e.g. was living for a month in a temporary residence, no receipts, didn't work anywhere and don't have papers to confirm that particular period in question. In some cases like that may be worth adding 30 days as you may be asked to prove your residence during entire declared period. If CIC/CJ would have doubts you've lived in Canada for 1095 days because you can't account for a part of it, you can be refused.
60 days would be overkill in the most cases. It depends on your specific context how much days you'd like to have over 1095 considering these aspects.