I have a no-name charger which works fine, although I paid more than $5 for it (can't remember exactly). If its powered by USB there will be no danger of a fire; if it plugs directly into the wall, then maybe so.
I also believe I've read that the iPhone itself "learns" your charging schedule, charges up to 80%, then shuts down in the phone until its almost the time that you usually pick your phone up, and then finishes recharging to 100%.
I don't see how magnetic induction could otherwise hurt your phone, but I'm an ME/IE, not an Electrical Engineer.