I don't even know if it's 'the norm', as I rarely garden in any particular traditional way. So all I can tell you is what I do and you can choose whether to or not lol.
I personally don't like the lower leaves on my tomato plants, as I find they get in the way when I spray (the underside of the leaves). So I snip them off...usually after the plant IS well established. I snip them at about a cm away fromwhere they join the main stem. I suppose I go up about 9 inches from the soil, or so, and I leave anything that looks like it has flowers and may bear fruit. I used to just throw those leaves away, or into the compost, but then I learned that tomato leaves make a wonderful organic spray for aphids, but still safe around plants and humans. Spraying the underside of the leaves is a must as this is where the little pesky stinkers congregate...on any plant. I only started using sprays (my own) last year, when I learned about the psyllid bug that was attacking tomatoes, potatoes and capsicums and that's when I found the one about tomato leaves. In addition, I make another two sprays, and I alternate when spraying, so that any pests don't get too used to one spray and become immune. The other two I make are a) garlic and tea tree spray and b) nee, oil spray. For the tomato spray, here's where I started, and I just adjust things depending on how many leaves I have available.
One to two cups of tomato leaves
Two cups of water
A strainer or cheesecloth
To make tomato leaf spray, simply soak one to two cups of chopped tomato leaves in two cups of water. Let it steep overnight. To make the spray, strain the leaves out of the liquid using cheesecloth or a fine strainer. Add another one to two cups of water to the liquid and add it to a spray bottle.