In addition to the observations offered by
@rajkamalmohanram a good margin over the minimum can reduce the RISK of RQ-related non-routine processing. No brainiac necessary to sort this out. The closer to the minimum the applicant is, the more closely the applicant's accounting of days in Canada needs to be scrutinized in order to positively verify the applicant did in fact meet the minimum requirement.
Waiting longer to apply can mean actually taking the oath sooner.
I would also add that there is no shortage of anecdotal reporting in this forum from individuals who were certain they made no mistakes, or no more than very minor mistakes, in their actual physical presence travel history, who later realize or learn they did indeed make a significant mistake. A margin larger than the sum total of mistakes will avoid the application being rejected (remember, IRCC has no discretion to grant citizenship to an applicant even just one day short). But the impact on potential RQ-related processing looms even far larger in this context.
Compare for example the applicant whose mistake is relying on entry stamps abroad to report dates exiting Canada (trans-Pacific flights, for example, can result in an entry stamp abroad that is two days later than the date the PR actually exited Canada; which at least in the past has been a rather common mistake by applicants), and was off just a few days in total . . .
-- for the applicant without a margin larger than the mistake(s), the application will be denied
-- for the applicant with a small margin but large enough to cover the mistake(s), the application will not be denied but taking the oath can be delayed by months or even many months, even more than a year, if IRCC issues a more probing version of RQ (Residence Questionnaire)
. . . with an applicant who mistakenly left out a three-week trip abroad (referring now to a particular actual case as reported by the applicant here) but who had a margin much larger than that, who acknowledged the mistake during the interview, explained it was an oversight, and was then approved and took the oath soon after the interview, no non-routine processing delay at all.