If we assume that this person has ample TFSA room., I would always suggest that monthly contributions go into the TFSA only. A transfer into the RRSP can then be made in February, with the TFSA assets,  up to the entire account value after doing the  income tax calculations that can only be 100% certain after the year end.

As a bonus, any gains that have been made throughout the year in the TFSA will allow for a larger RRSP deduction receipt than you would have if deposits go directly into the RRSP.

It is possible, however, that the RRSP receipts could be less in a year that your TFSA investments lose value. Having said that, it would be a fairly rare exception when that would happen.