Saving seeds is a great idea because it empowers the gardener to be able to continue generations of crop after crop withouth any external dependancies.

There is a science however related to sexual plant reproduction and the selection of desirable plant characteristics through genetic breeding. People spend a good deal of time hybridizing plants and selecting for desirable characteristics to be realized in potential seed stocks.

Genetic variation can be quite varied in seeds taken from your store bought veggies and since garden space is generally in short supply its probably a good idea to grow plants which have a better chance of displaying the desirable characteristics you are looking for.

I think that the seeds your have are definitely viable and you could produce some decent vegetables by using them, at the same time might be limiting your agricultural productivity by utilizing genetics of an uncertain makup.

It's a shot in the dark.

Saving seeds is a wonderful practice- all the diversity of crops like tomatoes, apples, nuts,wheat, rice, sunflowers...... all came from gardeners going out into the fields, generation after generation, and saving the seeds.
Pragmatically, though, some things are easier to save than others. Some things are just not viable for the home gardener. A lot of it has to do with the sexual proclivities of each species. Tomatoes, peas, and beans are the easiest of all- that's why there are so many kinds. Those are easy because they are (mostly) self pollinating, and they don't suffer from inbreeding pressure (very much). So if you start there, you will have lots of successes.
Squash has an insect pollinated flower, so if a squash is not grown for seed (by hand pollinating), or grown in a really big field with no other squashes around, it will end up crossing with another squash, and you stand a really good chance of getting an inedible gourd.
Corn is a great example of a wind pollinated crop- it is easy to save the seed if you are all alone with no neighbors for miles (and miles and miles) because the pollen can blow that far. I have heard that native corns in remote Mayan villages has shown significant genetic contamination of genetically engineered corns from the States.
A twist on all of this is that most of the commercial crops you can buy at the supermarket (and the home garden seeds you can buy) are hybrids. Hybrids are crosses of parent plants that grow better inall sorts of ways- vigor, bigger or sweeter fruit, more yield, et cetera. Problem is though, that the children of hybrids almost always revert to the qualities of their humbler grandparents. So that big red tomato that you bought at the store will often have seeds that grow into scraggly little plants with boring fruits.
Saving seed is an adventure- it can be complicated or easy. I would recommend starting with the easy crops like toms and peas. Especially snow peas- they are so expensive to buy, and so easy to grow. Lettuce is another great one to save, but you don't buy lettuce seeds at the supermarket, you buy the leaves. The other recommendation I would make is to start with good seeds- one that you know what variety they are, and that they will come true- that they are open pollinated, and not hybrids. I make that recommendation because to go to all the effort of starting little plants from seed, and caring for them as seedlings, and growing them all season, you really want to get something that is tasty and worthwhile. And then you will want to save the seeds again, to start all over. God knows I am stuck with all sort of seeds in my collection that I have to grow every few years. I'll never be able to stop now.
I also recommend Susan Ashworth's book _Seed to Seed_
Good luck!

I started my garlic this way... bought the biggest clove i could find, and stuck every clove in yogurt containers... they're happily kickin' it in my front yard looking like two-foot tall grass.