I seem to get leaf roll on a few plants every year. I ignore it because there is not much you can do.
It is pretty common to have some spots (septoria and early blight) on older lower leaves, for various reasons. When a leaf looks to be about half way infected-killed off I will remove it, and if around 20-25% of the plant's leaves are infected I will consider spraying, but a lot of that is dependent on the weather. I will clip leaves and spray if a plant looks like it is getting worse and the weather is going to be wet. I won't spray if the weather is going to be hot and dry.
Young plants depend on the older lower leaves for their growth so I try to leave them on until the plant gets some size to it. How different people handle the situation depends on their experience with their plants in their climate, and unfortunately that experience comes from watching plants get screwed up at least one time rather than doing all they can to keep plants picture perfect.
Unfortunately, with the late blight already starting up this year people will have to be extra dilligent about treating problems quickly, and with a firm hand. Luckily Late blight does not look like the other foliage diseases.
This link will lead you to some good pictures of diseases