Transplanting seedlings to next bigger pot
Often when I transplant newly sprouted (tomato)seedlings the length from the very bottom of the root to the first true leaves is greater than the depth of my next pot. Is it better to curl the roots in order to get more of the stem buried? Or is it extremely important to NOT curl the tap root around the bottom of the pot?
No, it is not extremely important to avoid curling the tap root. I usually transplant up to the first true leaves and let the tap root fall where it may.
I'll make a couple of further observations. First, you could use larger pots, giving the tap root more room. Second, it is quite early to be even planting tomatoes, let alone transplanting, unless you've go some season-extending method.
By the way, the efficacy of transplanting tomatoes deeply is based on a 1996 research article from Florida: