The general feeling regarding application rounds that all except the last round have fairly equal chance. My view on the rounds is that R1 is a trial round and may offer some advantage. Since the quality and volume of applicants have been increasing, school does not have a good feel the year's application at R1.
School can over or under estimate the quality/volume in R1.
1) If the school underestimate in R1, a less qualified applicant can in theory get accepted.
Advantage applicant.2) If the school overestimates in R1, I believe it still take the conservative approach and waitlist the applicant when in doubt. The applicant is then re-evaluated in later rounds, thus compared with a more complete picture.
In this case, the applicant is not disadvantaged.Thus, in my opinion,
it's better to apply early if possible. However, this does not mean you can cut corners for R1. You still need to put together the best possible application. If you believe that your later application (not last round) will be much better, you should apply later.