Tomatoe grafting vs growing heirloom tomatoes w/ large root system already....
I start my tomatoes from seed early inside, + transplant them into progressively deeper pots (by clipping off lower leaves + burying tomato stems deeper to have a greater root systems). When I finally plant them outside, their roots fill at least 6" deep of a 1 liter soda bottle. I read about grafting tomato plants on to a good rootstock tomato for better production. However, I just learned that the grafted tomato plant should be planted with the graft site above ground. Wow, I would be planting the grafted tomato with a smaller root system......but better root system???? What do you think? Is this really worth it? And I bought the rootstock seeds....very expensive - about $22 for 25 seeds!!!
For most home gardeners, we don't recommend it. It is difficult to do, and the success rate is low. Special set-ups (growing chambers) are used. As long as the home gardener has been adding organic matter and has good soil--and no serious chronic disease problem, such as fusarium, that can't be avoided--then it isn't worth the effort and cost. Outcomes are as good in a typical garden.
For the curious, we recommend first purchasing some plants and experimenting with them.
If you decide to use your purchased seeds, there are good tutorials online, such as at Johnny's Seeds. We'd be glad to hear about your experience.