How you deliver it will depend on the places to which you are delivering. Most places are set up to project 16:9, so a letterboxed project is the simplest way to deliver. Often, if you deliver something other than 16:9, it will be stretched. However, if they specify that they can accept something in a different aspect ratio, you can deliver a render in that aspect ratio.
Most often you will either deliver a 1920 X 1080 standard source file or you will render to a specific codec using specific settings that are called for in a set of delivery specifications. Make up your own delivery specifications or use someone elses and you are going to be in just guessing. Standard HD is always safe. On occasion you can deliver 4K, but here again, you have to ask the folks that will be projecting the image about the format and follow their recommendations exactly.
The most important part of the production process is making sure that you do not distort the original footage. If it was shot full HD in square pixels then masking is the way to achieve your Cinemascope look. If it was shot using a camera that uses a non-square pixel format (DVCPro HD for example) then you would still work with a square pixel comp but, and this is the important part, you would let AE automatically fit your non square pixel footage in a square pixel comp. Manually changing the pixel aspect ratio of the source footage will foul things up and distort the image.
So, before we can give you a specific production workflow that should be followed we need to know exactly what kind of footage you are using in your project and exactly what kind of system will projecting the final image.
I am shooting a short film for myself in 1920 x 1080 (Nikon D800) and once it is finished, send it to various film festivals for them to screen and to get exposure to the film.
I wish the resulting rendered film to be in a format that is the most useable without the need to adjust it any further. Of course, if a big opportunity presents itself, I can re-render the film in a specific format.
This will be my first film, so I wish to do as much as possible with best practice in mind and appreciate all the insights you are giving, no matter how trivial they might be to you. I am open for any tips and wisdom.
Just work everything in standard 1080 square pixel comps. Set up a masked layer to achieve your widescreen and lay that over the final output. Keep it simple. All film festivals will accept standard 1080 video.