We all do things different, but you know what, I have never owned a flexible block. When all those mile long blocks came around, I never bought one, never saw a need. Now I am a tool nut and have bought many tools that I hardly use, I buy the best, (mostly) nothing but the best profesional tools. And I don't own any of those super long blocks or flexable ones.
I have done a ton of vintage cars and VW bugs, tons of them and never used one of those blocks getting those small curved panels like a Bug fender to the large panels like a hood on a mid sixties Chevy with zero waves or funks without any of those blocks. I look at the VW bug or 50 Chevy pickup fender as a "curved flat" if that makes any sense. I see no difference in blocking out a high spot in a flat panel or blocking out a high spot in a curved panel. You have a high spot you feel, you block it out so it's gone, never saw a need to think any different.