I must tell you that you are doing a great job of taking care of your parakeets!
I think your parakeets may be going through a normal molt since it is springtime. Do you see pin feathers on them? If so, then I would not worry about them unless the extra preening and apparent itching continue after the pin feathers are gone or the situation becomes worse.
If you'd like to go ahead and check for feather or blood mites, it is easy enough to do so. First, place a dark piece of paper in the cage bottom at night. Check first thing in the morning to see if there are any light spots on the paper which move. The next night, use a white paper and check the next morning for red specks that move.
If you find either type of parasite potentially present, a vet visit is in order. Don't use over the counter sprays since these can harm the birds and are not very effective. Don't use a "mite protector" either since this contains toxins.
The only dietary improvement I suggest is more fruits and veggies and less seeds, plus adding a cuttle bone. Parakeets should have about 35% seed and it can be hard to get them to accept that low amount of seeds, so change slowly over time to avoid stress. Just offer more fresh foods and a bit less seeds. All parakeets need cuttle bone for health and also direct sunlight or a supplemental light that is full spectrum to produce the effects of the sun so they can absorb the vitamin D from the cuttle bone and other foods.
If I can help further, please use "reply" after accepting this email. I'm adding a few helpful links for you.
Locate an avian vet in your area: http://www.aav.org/
Parakeet Diet: http://www.lisashea.com/petinfo/articles/bird_food.html
About mites: http://www.exoticpetvet.net/avian/mites.html
Household dangers: http://www.parrotparrot.com/birdhealth/alerts.htm