no plug is 100% safe but remeber that the plug will be pushing itself tighter as the tyre gets pressurised. Ive had a few done myself and I have one in my back tyre right now, I aint throwing away £120 for nothin' however you may find that some people wont repair because they dont want their name to it incase it goes Pete Tong, fair enough I say but as most posters have said, there are people out there who will do it. The way you describe the nail position I would have no hesitation about a plug but its all about choice, if you cute enough you could always plug it and sell it to offset cost of a new tyre