An all SEED diet is incomplete. Which is why it is recommend that you buy seed and grains, since cockatiels are granivores. Parrots have been shown to benefit from pellets when compared to a seed only diet. However, a mix of seeds, grains, legumes (including soaked and/or sprouted) and veggies would be the BEST diet, even if it is inconvenient for some.
Did you join feeding feathers and read the articles for budgies and cockatiels?
The site has a ton of great info and recipes for mashes too. A mash helps get good things into your birds diet that they may otherwise pick out of their bowl.