First let me say there is a FAQ here about this question as well as many discussions about it the search will pull up for you.

Feeding seedlings is a very debated topic, some do, many don't. Lots of variables to consider. The primary one being what nutrients are in your growing mix? You'll have to check the label for that info. If it contains added nutrients then nothing else is usually needed.

Another variable is how long the plants have to go until transplanted out to the garden. If tomatoes were started at the usual 6-8 weeks before transplanting nothing may be needed. But if they have to sit around for 10-12 weeks before transplanting they will need feeding.

There is no "what most use" fertilizer. There are 100's of choices and the choice is yours. While the plants are in containers most organic fertilizers will not work as there is no active soil food web to convert them to a useable form so liquid synthetics are used. If you feel you must use an organic only then one of the many fish emulsion/kelp blends or some sort of compost tea or worm tea are really your only good choices. And yes, they do smell. That is the nature of organic fertilizers.

But if you do choose to feed them then wait until AFTER they have developed at least their first set of true leaves, use a liquid not a granular form, and dilute it to no stronger than 1/2 strength (many use 1/4 strength). Just as you wouldn't feed a newborn baby adult portions or steak and potatoes, the same holds true for seedlings. :)

Dave

So i start fertilizing at half or little under half strength as soon as i see true leaves forming. and i water with a diluted to 50% mixture every watering i have gotten awesome results. not for fertilizer i use a Organic Doctor earth fertilizer for veggies. Then i also use a liquid fertilizer called florolicious plus it is made of organic fermented beet juice and kelp. it is very mild on stinkiness when you delude it to 2- 2 and 1/2 teaspoons on one gallon of water. really has not smell. I have some videos on my youtube channel where i show these i need to still make a video on mixing fertilizer's. but i will do that in do time hopefully soon.

Here is a link that might be useful: TheItalian Garden