Is jailbreaking on already used iPhone (apps installed and stuff) safe?

Hello, I’m wondering what iPhone should I look for. I know I can’t restore and downgrade to anything earlier than iOS 7.1. I also know that the OS shouldn’t be installed though OTA upgrade, but I’m not sure if I’ll be able to jailbreak successfully a already used iPhone that has been upgraded to iOS 7-7.0.6 and then used. I wouldn’t be able to restore it again to 7.0.6. So, can that cause any issues or the only thing that matters is for it to be upgraded through iTunes? Regards