This one is simple, you might as well tattoo it all over your face, because it鈥檚 very likely you鈥檙e not doing it:
Keep your seeds.rb information up-to-date with your data model, for Pete's sake.
Make the next guy to work in that project鈥檚 life easier by not having him editing the seeds.rb file as he spends like two hours invoking rake tasks to setup database and getting fugly errors in the terminal one after another.
Be the good guy. You don鈥檛 know when you鈥檙e getting to continue other鈥檚 work in a project, and you that b-word they say about karma.