The thing with verticals is, that in full-space, wide-angle view, it's never good idea to mess with them. Your brain would be correcting it, architectural photographers are correcting it for decades, it's the correct look we've associated with internally.
Experimental-wise, you can dislodge them about 5 +/- percent from complete straight, this way, even with digital image, your brain will try to correct it and it will end up looking sort of more natural if that's what you're after.
This is a pretty in depth response which I really appreciate Juraj. I tend to experiment with these values on more of a "it feels good" basis so I am thankful for the numbers you've given me.
Naturally the goal is always to push forward with ideas and compositions but still stay in the so called "normal" zone.
@Rhodesy
Well actually I started using C4D first and the way things work in there make my brain feel quite comfortable. Switching to 3ds Max (for a few projects) wasn't that big of a deal but I did have to put a lot of hours into getting up to spec on where all the buttons are and what is the best workflow for this and that. Digital Tutors really helped with that with their Introductory courses. It is good to remember that if you do "master" a certain part of the 3D workflow then switching won't be that big of a deal.
I switched (and then back) because I felt like I was missing out on rendering options in C4D. For example, the V-Ray we have is still the 2.5 version without the GGX BRDF. It was even harder a year ago when Corona was barely at V1 of its C4D implementation so of course I wanted to see what all the fuss is about. Also, I felt like I hit a bit of a ceiling in my technical knowledge and doing tutorials from scratch kind of makes you notice a few things you can improve in your workflow.
As to the comparison between working in 3ds Max and C4D as a C4D user, well... I can't say much else than that both programs have clear advantages and disadvantages. Just to name a few highlights, modeling in 3ds Max has way more options but its UI tends to freeze a lot and generally just feels clunky. C4D on the other hand doesn't have such robust tools for modeling, the rendering engine options leave a bit to be desired but doing workarounds and reinventing the wheel feels much much smoother in C4D due to its UI and parametric nature. Plugins, boy 3ds Max has a ton of those (and a loooot are fantastic) but then again I found myself installing plugins for things that were already implemented in C4D in an easy to use way.
I guess it just depends to what feels more comfortable in the end. Hope it helped :)