While it's been a while since I've used strips, when I did I used 6 round strips and loaded 4 rounds.
My logic for doing so:
I have 5 chambers. With the 1st pair of cartridges I've got a 2-in-5 shot at getting those rounds chambered. With the 2nd pair of cartridges I've got a 2-in-3 shot at getting those rounds chambered. I can do that pretty quickly. If I had 5 rounds in the strip, that last round has exactly one chamber it can go in, which can be tricky under stress and takes longer to do than either of the previous pairs because it requires a little more precision. As soon as the rounds are chambered I drop the strip, so a 6th cartridge would just end up on the ground once I loaded all 5 chambers. By having the space in between the pairs it gives me a little more control and a little less likely to fumble the reload. By gripping it as I do in the photo I can get a good physical index between my hand and the cylinder to get the ammo loaded. This lets me reload without looking at the gun so I can keep my eyes on my surroundings.
So, my priority is to get the gun back in action quickly if I need to reload, and with as little potential for fumbling the reload as possible. After some experimentation with different combinations (6 rounds, 5 rounds, 4 rounds in different arrangements, etc.), I settled on 4 rounds in the strip. It works for me. I use speedloaders now, but I still practice occasionally with strips in case I need reloads with a lower profile.