Does inflexibility in specifying execution date of ETF savings plan have any detrimental effect? [duplicate] (opens in new tab)

My broker, and afaict quite a few of those that offer low/"no"-cost ETF savings plans, suggest or even demand that execution dates are specified with fairly limited granularity, e.g. a choice between operating day on or following the "1st of the month" or "15th of the month". Makes sense for low volume to combine as many customers plans as possible, so they can offer investing at a fixed X,00€ rate, then credited as owning fractional shares. However, I learned elsewhere that advance knowledge...