This section is from the book "Everywoman's Canning Book", by Mary B. Hughes. Also available from Amazon: Everywomans canning book; the A B C of safe home canning and preserving.
All canned vegetables should be heated through to the boiling point before being served. In home canning, where preservatives should never be used, this is a matter of wise precaution, that any bacteria which may have developed may be destroyed. Scientists who have made special investigations of home-canned products inform us that there is not the slightest danger of "botulism," a rare form of poisoning from eating home-canned goods, if the product is first boiled for a few minutes after being taken from the jar. This does not mean that products must be eaten while hot. In the case of canned vegetables to be used in salads, they may be boiled and set aside until chilled through.
The flavor of all canned products is greatly improved when the jar is opened six hours before serving. Pour the contents of the jar into a large bowl and set aside in the icebox. Oxygen will then have permeated the product before it is served. Vegetables requiring a long processing period, like corn, beans, greens, etc., are especially improved in flavor if allowed to stand out of the jar for six hours before being used.
Jars are sometimes hard to open. This way is recommended: Using a thin knife blade, such as a paring knife or pen knife, insert the point under the lower edge of the rubber and press firmly. This will usually let in enough air to release the air pressure on the top of the jar. After several attempts, if this is not successful, place the jar in cold water in a deep dish. Have the water come over the top. Bring to the boiling point for a few minutes. The jar will then open easily.