Well, I rarely do that. I find that new stems are extremely problematic as new, green, pliable stems will bend and flop a lot in macs and smooth hydrangeas after summer storms -when the heavy mophead form blooms are open- so, I prefer not to prune any stems, especially the older, stronger and woodier stems... unless they are bent and will not go back up on their own. I let the green ones grow as they want but, once they get about the max height per the plant label, growth on those once new stems diminishes and they are relegated to being slow growing woody stems. No difference here with air and health, including the cercospora infected ones. But try it anyway if you want.

A handful per shrub - 6 new stems on average with a bloom and a stem thinner than a pencil are weak ones I will cull out once woody when I see them in spring. But those are the obvious ones - straight floppers as you describe. But for all the new wood with bloom, while they only have a single bloom on the terminal end, they ain't floppers.

I think partially in my mind, I'm also trying to avoid the super grown growing base like some of the old Mac's I removed. We're talking a good 36-48 inch all bark mass. Beyond woody is how I would describe it.

mr. e. scissorhands will put the pruners down and walk away slowly

I was thinking more air, less congestion, more energy for younger stems and as far as the look, honestly, once it's leafed out and it's a ball of green and blooms, I don't think a few missing stems would be noticed. I guess extra stems/more stems can serve for backup for winter damage

I lost some stems in -full-, and others were cut in half due to snow load / snow decay (aka, some stems were buried for too long and were mush this season). Didn't notice the miss stems once leafed/bloomed out

I agree with gardengal. I learned the hard way. I would prune in the fall and get buds but no blooms. Finally, I decided to simply leave the plant alone and low and behold, ever since, I have a healthy shrub that blooms year after year.

Sometimes they just need trimming back to keep them in bounds. At our last house I had a blue one that crept out for several feet to where you couldn't even walk around it from the back yard to the side yard. I would cut it back, dig out the new plants and give those to neighbors. It seemed like I would have to do this every 3 to 5 years else the wife would get annoyed.

It often would be close to the border line.

At my current house I rarely trim any hydrangea except this year I took hedgers to my oldest ones (Lady in Red and a cutting from the blue hydrangea above). Cut them to about half the size and the new growth looks good thus far. The blooms were not as plentiful as they had been in previous years and the spread was getting to be too much.

Wow! Yours is enormous compared to mine hc! Simply gorgeous! Mine is still pretty much in its infantile stage. I will certainly keep that in mind to trim as needed!