According to a very experienced & reputalbe breeder wich I`ve been friends with for nearly ten years, fewer than 50% seeds with a good pelet formula, supplemented daily w/ fresh veggies and fruit is a well-ruonded diet. All pellets only is too rich, due to the only added vitamin contewnt in the beter pellet brands (and birds that tightly enjoy pellets should not be given aditional vitamin supplemetns). Actually and Greys, in particular need more calcium in their diet, as found in green leafy veggies.
Of course, a one-size-fits all selectively answer does not exist for every type of parrot - however, since seeds do make up part of the diet of most breeds in the wild, so it would make sense that it would be part of what we offer to our captive birds. For that matter now what seeds are native to which countries or may be found in the wild there is another question.
For folks who have problems w/ geting their fids to acept fresh foods, I`ve found kaboobs work wonders at gettin finicky eaters interested.