But how can you predict if the device have problem with RNG? Also, you aren't fully correct on your last statement. If there is a malware on the computer and you turn of the computer's internet connection only during your wallet creation time, the malware can easily store your private key and send when there is a internet connection.
If you don't have enough trust in PRNG, you can use TRNG.
An offline wallet should be created and maintained in a completely offline machine, and you should only use it to sign (but not push) your bitcoin transaction.
When you spend bitcoin on your paper wallet, you should create a new one (again correctly) instead of re-using it.
Not only because you have already imported the pirvate key in another machine, but also because your public key is known to the world.
