nowaday mixdowns are a mix as you go affair. Generally as i'm writing i make sure i get the sound i want from the jump. Tweak it and continue writing. The idea is that when you're finished writing the tune all you need is a tweak here or there when you switch to different listening mediums.
Multiple sets of different sized monitors are not a necessity but it's an amazing time honored method of working and checking your mix by having them. I personally use 4 different pair. A pair with 15" drivers, a pair with 9", a pair with 6", and one speaker(for mono checking) with a 3" driver. The idea is that if the mix sounds correct on all those speakers without tweaking...then your mix is dead on. You've basically covered your ass.
I personally love to make sure that the mix is thumping on that 3". If you can make bass come through on that thing then you can be sure that mix is on point. Hint: mix and adjust volumes at the lowest possible volume setting without the monitors being off. Try a kick and a hihat at first. Make sure the hihat volume is all the way off and slowly begin to mix it in. You'll notice immediately when the hi hat is too loud. The idea is to sit it in there just barely below the kick. Once you're done switch to the main monitors and turn the volume up. It should sound right on point. People fight with this because they try for hours while the mix is blazing not noticing that ear fatigue begins to sit in and messes with your perception of sound and high end.
hope this helps
"you bust your ass to make all the money you can...thinking it solves shit...and then you realize..when and if you get there at all...you got more problems and you missed out on the one thing you can't buy or get back...your life! Spend time with friends and family...You can be broke in other ways too!...."