For reading websites I always rotate my phone to landscape,as it makes it easier to see, and the content generally fits better, most apps I use in portrait mode. My tablet is always used landscape, don't think I ever use it in portrait, although I did when I used to game on it. It is extremely annoying when a app doesn't auto rotate, or not quickly enough.
My eyesight is a lot worse these days and I have to use glasses when using my phone or tablet, I know some people that just increase the font size though. Sometimes I find some apps or websites where the choice of colours makes it difficult to see.