I use an 81A only occasionally with this film, mainly when shooting in deeply open-shaded environments. However to use it or not is largely due to personal taste; some people prefer to allow a colder cast to the end result when appropriate, or some people really like a warmer cast to everything (even using an 81B for example). Personally I find even the 81A is too much in many cases.
Thank you, exactly what we were wondering for a shot we plan to try for a shaded area. Been a long time since shooting E-6 and its fun, in a learning a totally seemingly new way to expose. Will give it a try and see the difference with a shot and shot without, hopefully at correct exposure. ;-)
Back when my go-to film was VS (RIP), I typically used a warming filter with Provia. Putting Provia and VS side by side revealed completely different palettes.
Now the Provia is normally shot unfiltered as folders are not as filter friendly as 35mm. I've come to embrace it for its own qualities and prefer its saturation to the new Ektachrome (but we still love Ektachrome and give thanks to the Great Yellow Father).
Sometimes the blue cast in shadows and overcast situations works in your favor, sometimes it does not. With Provia, it's a matter of personal taste. You can always do side by side tests with and without an 81A. But the effect of UV on color, esp at high altitude, is a different issue, so I recommend at least having a 1B skylight filter on hand.