If I understand correctly what you are asking is:
When solving a resistive divider equation, does it matter if I select R1 and solve for R2, or do I select R2 and solve for R1?
And also arbitrarily, R1 is the leg that goes to ground, and R2 is the leg going to the voltage being measured.
Short answer, it depends. It depends what you want to optimize.
For instance, let's say you want to measure a voltage, loading it with no less than 1 Meg.
In this case, it makes sense to start with R2 = 1 Meg and solve for R1. The equivalent resistance will be larger than 1 Meg.
On the other hand, the ADC's datasheet requests that the source impedance be no larger than 10k, then you select R1 = 10k and solve for R2. The equivalent resistance will be now lower than 10k.
Like everything else, there are tradeoffs, and you will find yourself re-calculating the original divider thru several iterations.
In this instance, Excell or any other spreadsheet is your best friend.