I plant 2 or 3 seeds in each cell - basically hedging to ensure that there is a plant in each cell of the 4 or 6-pack. Should all the seeds germinate, I wait until they have their true leaves, then I choose the best, and murder the rest! Rather than pulling out the weaker plants, which could cause harm to those I want to keep, I simply cut them off at the soil line and put them in my compost.
And, for a gal who is starved for warm soil and real gardening, I have to say one of the best parts of this slaughter was the wonderful tomato smell I got, when I took the scissors to them. Summer just can't come soon enough.