I did the first pruning of my basil plants some weeks ago, and all of them have branched out healthily. But in reference to the main stem (the original, straight stem that I pinced earlier), I saw two different results. Some have grown the main stem back and some others don't. Which one would I prefer? And what makes the otherwise happen? Should I pinch it back or leave it as it is?
A photograph showing their growing conditions might have been useful, but to some extent, it depends on where they're growing as to whether the mainstem continues to grow, and also quite where and by how much you pinched it out. If the plants are close together, those which receive less light to their lateral (side) branches will grow upwards in order to maximise available light. Equally, if what you pinched out was only a tiny amount, then its possible the cells which dictate apical dominance were still present, even if reduced, and the mainstem continued to dominate; if the plants which are still showing apical dominance have reduced sideshoots, likely this latter is the explanation.
I can't say whether you should pinch back the mainstems again, because it is dependent on their growing conditions and how crowded they are - generally, you would reduce the mainstem to encourage bushiness and lateral growth, but if the plants are a bit crowded, this wouldn't work too well.
You WANT vegetative growth, not flowering. If your plant isn't flowering then great. You want to use higher nitrogen in relation to phosphorus and potassium. Pinching apical or terminal tips is all you need to do...once or twice. That is where most of the energy in the topgrowth of your plant is poised. Cut that off and that energy is more evenly distributed to lateral, vegetative points. Do not over fertilize. Use your basil or harvest and dry. These aren't long-lived plants. Don't worry about 'main' stem or branch stems. Focus on just the tips and WHEN flowering starts keep pinching flowers back. This is why one should plant new seeds every few weeks to lengthen availability. Send pictures as this is just guessing without!! Grins!