I’ve been looking for solutions to produce my own public and private key set for alt coins for the purpose of secure cold storage. bitaddress.org, liteaddress.org and the iota wallet generator all allow this by generating keys that can be edited then re-verified, done offline. Basically for the most secure cold storage I do not trust the keys that wallets generate for me and would like the option to change the numbers and create a working address from those altered numbers. A secure custom generated key set offline would be the most secure way to story assets.
I personally wouldn’t get into the business of editing numbers and re-verifying just because those numbers aren’t any better than the completely randomly chosen numbers these programs produce. But having said that, if you don’t trust either of these websites, then download the source and generate your keys offline. That’s what I do and am happy with creating keys with this approach. There’s far more security to be gained from doing an offline key generation than editing the numbers generated manually. The offline approach assures the public site wasn’t recently hacked and malicious code injected to steal your private keys.
If you’re not a programmer and thus cannot verify by reading the source to see that the code’s not sending your private key out to the world somewhere when you generate it, then simply pull the network plug to “air gap” your computer while you generate the key pairs and move your private key file off your computer onto a USB drive or print it or whatever before reconnecting your computer.
Point is, it’s more important how you handle your keys and guard against theft than the actual numbers themselves that leads to the best security practices overall.