In response to the OP: it really boils down to where and what sort of fighting you plan to do. As Team Western Allies, I generally try and bulk up to 5 brigade division ASAP, as my usual primary Theater will be Western and Central Europe, which has many large ports and generally great infra, and the German units are going to be no slouches, so every advantage each division of mine can get is critical. Especially as the Americans, if I play historically and don't start a proxy war to grind XP for my unit commanders then my leaders will generally be far outclassed by German ones so I need to ensure my individual divisions can go toe to toe with German ones. I love the SM build (ARM/3xMAR/ENG) as the US and as the US has IC and LS to spare, I sometimes make them my "line" divisions and use my "armored" divisions as ARM/MECHx2/AC/[AC/TD/SPART] (depending on whether I want a specialized or fast division). I personally think people get way too concerned with maxing the CA bonus and just make relatively unwieldy divisions that a sleeker division that might lack a 5% bonus (easily overcome) from its total CA amount would beat handily.
Of course, when playing as the Germans or Soviets I use 4 brigade divisions, as I will almost certainly be fighting in poor infra situations, have a larger front to cover, and will also have to shepherd my officer ratio somewhat (especially as Soviets), and 5 brigade divisions in a quantity sufficient to fight Barbarossa will eat officers swiftly. Also, I'm more prone to using less supply/fuel intensive divisions (I'll even sprinkle some 4xINF for doing dirty work in shitty areas), so I tend to use a lot of 3xfrontline/1xsupport divisions. Try running an army or two of 5 brigade divisions into Moscow and you will run out of supply or fuel, guaranteed.
In general, adding a 5th brigade to your builds specifically would make them either more durable or have a greater concentration of firepower at the cost of each div being more logistically intensive and officer heavy. However, not using 5 brigades is a completely viable option as well, and really boils down to personal preference.