it doesn't "really" matter

most people will argue the fact that you are probably using more fresh fluid when you start at the front since you begin to pull some of the fresh fluid from the front, all the way to the back before getting all of the old crap out.
they will also argue that if you start at the front, there is a higher chance of getting air back into the lines because you are "going over" what you have already done. The idea being that the rear typically start at 1 line coming out of the BMC so it is less likely to suck up some air once you have flushed them through.

I don't know how much I believe in all of that. I don't know that it really makes a difference either way. I have always done it from the furthermost point first just because it gives me a pattern to follow. But I also go back and do each one a little bit more once the whole system has been flushed, just in case.
for the average user, using the valvoline 3&4 brake fluid (which is the absolute best "cheap" fluid you can get) It doesn't make much difference. But for the heavy user, that has a need for better brake fluid (i personally use superblue mainly because it has the temps i want, and you can get it blue, or gold, which makes flushing the system easier, in my eyes, just alternate colors) then making sure you get all of the old stuff out is a major issue because just a little bit of old fluid can dramatically lower braking performance.
but like i said, for the average user, just making sure the air is out is good enough.