I am starting to entertain the opinion that no additives to the sterile, soiless mix should occur.
Tomato seeds started in , then reared , until time to transplant directly to native soil mix,...should not be added anything unless symptoms exist, which is rare in the above idea.
Some curling is natural in some varietys as I have noticed.
Overwatering creates alot of trouble in the inital phases........, others with more chemistry, and biological knowledge may be better suited to answer questions of mag levels and what properties soiless mixes have or may need.

Tom

Whether 1 tsp of epsom salts to the pot is too much or not depends on the size of the pot but it is water soluble so flushes out easily with watering.

If the rest of the plant looks fine, likely it is just a temporary response of this one leaf to some sort of stress and it will correct itself. Plant leaves do all sorts of things in response to their environment and most of them are no problem for the plant. You might want to check the undersides of the leaves for aphids and/or spider mites but otherwise I honestly wouldn't worry about it.

Dave

I have one variety 'Fireball' , where all the leaves curl under slightly. I figure it is just their nature :)

Thank you all for your responses! This is the only young tomato plant that is doing this. I have two, both started from the same seed packet. I did add the ES to both pots when potting up, but I think I may have put more in the curling one?

Also, the curling seems worse at night, before I go to bed. At that point, all three stems have some curling, the bottom stem is the worst. In the morning, only the bottom stem is still partly curled under.

I'll wait it out and see though. :)