Navi Hack

Hi new member here. I know this has been covered probably thousands of times and, based on my past forum experience, some forum trolls will be upset because I didn't just read through them. But, some have been up so long that the waters are muddied. So I apologize in advance. Here are my questions:

Is hacking the nag screen safe? Does it have side effects? If I screw it up, is it a costly repair and is it even possible to screw up for that matter?

Thanks for your time.