|The best place to mount a fan on the back is as high as you can mount it.|
The side of case fans is often marked with an arrow showing you the direction of the air flow. In any case, it sucks air in on the side where you can see the entire fan blade, blow airs away from the side that has a support strut and a non moving center part in front of the fan blade.
A side mounted fan for a duct that is directly aimed at the cpu heat sink on the cpu should blow towards the heat sink (the cpu fan blows towards the heat sink if it also has that), but usually you don't need one.
Don't run more case fans than you need. Too many will just suck more dust and lint into the case interior and you will need to clean that off more often. If installing a case fan or an additional case fan does not significantly lower the cpu temp or the temp inside the case (e.g. mboard temp) shown in the bios Setup, you don't need it.
If you have a desktop case near or on a floor that has wall to wall carpeting, the dust and lint will collect a lot faster.