Hi Michael,

Thank you for contacting the UNH Extension Yard & Garden Infoline. My name is Debi Green. I am a Master Gardener volunteer and am happy to help answer your question!

It is my understanding that the suggestion for planting tomatoes at an angle is a technique for if you have leggy transplants or your more mature plants have become leggy. It is not necessarily better one way or the other for tomato plants in general. See https://extension.umaine.edu/publications/4311e/17-stage-3/

"For most plants, plant them as deep as the roots currently are. The soil surface would be even with the soil surrounding the transplant, do not bury the stem. Tomatoes are an exception to this rule; they can be planted deeper—up to 2/3 of the plant being underground. Tomatoes have the ability to produce additional roots on the buried stem."https://gardening.cals.cornell.edu/lessons/project-s-o-w-seeds-of-wonder-food-gardening-with-justice-in-mind/unit-3-sowing-seeds-of-curiosity/3-7-planting-the-garden/

My personal experience is that I have worked in a garden nursery that sold both cell packs and tomatoes in 5-10 gallon containers and they were always planted straight up. So I don't believe either one is better, it just depends on whether the plants are leggy or not. 

I hope you have found this helpful, and please do not hesitate to reach out again with any future questions. We are always happy to help!