Canning techniques have changed quite a bit over the years as we have learned more and more about food safety. And while many of you may still can "the way grandma did it," we don't want anyone to expose themselves to potentially deadly botulism. Luckily, tomatoes are considered a high acid food, one that resists botulism more than most. Still, the pH can vary widely and it's better to be safe and sorry. So, back in the 1980s, the US Department of Agriculture changed its recommendations for canning tomatoes.
Current USDA Recommendations for a Boiling Water Bath:
1. Use only firm, ripe tomatoes. To loosen skins, dip the fruit into boiling water for about half a minute; then plunge them into cold water. Cut out the stem ends and peel the tomatoes.
2. Quarter the peeled tomatoes and bring them to a boil. Pour boiling-hot fruit into clean glass jars, leaving a half-inch clear space at the tops of the containers. If tomatoes are very ripe or bruised, add 1/2 teaspoon citric acid or 2 tablespoons lemon juice per quart. Wipe jars with a damp cloth. Secure lids.
3. Put filled jars into canner containing boiling water; add more boiling water if necessary to bring liquid an inch or two over tops of containers. (Be careful not to pour water directly on the jars.) Process hot-packed tomatoes for the following times, starting when the water in your canner comes to a rolling boil:
Pints: 35 minutes
Quarts: 45 minutes
4. Boil gently and steadily throughout the period; add boiling water if necessary to keep containers covered. Remove jars immediately when processing time is up.
For more information, refer here to the USDA's guide for Home Canning: