It depends on the site, and the person. But I almost always hunt with a friend these days.
Old house sites, older parks, etc I have no problem taking people to. There's enough old houses to go around, and I love seeing other folks make cool finds. It's the Civil War sites I'm particularly picky about, for a couple for reasons.
First, like Terry said about his gold sites, they're a non-renewable resource. Once those sites are cleaned out, they won't ever come back.
Second, they take a LOT of research to find. And I'm not talking battlefield sites, those are easy to find, it's just a matter of getting permission. They've been hunted for 50 years already, and I love hunting them with other people.
But the camps and smaller skirmishes can be tricky to track down. I might go out three times getting nothing before I finally nail it down and pull some minie balls. If you're not willing to put in the work to go out with me every time and get skunked repeatedly when we try to find it, I'm not going to take you with me when I finally do find it. Also, you have to be willing to share your research as well, it's a two way street.
Third, I want to make as sure as I can that these relics are going home with people who value them for the true meaning of the history they represent. If you toss all your bullets in a box without regard, or you're just in it to get the most and the coolest stuff, I'm probably not going to enjoy hunting CW sites with you. I document all my hunts, and organize my finds by location. I enjoy knowing where things came from, who they were dropped by, and their significance to the overall picture. Even though it may just be a 3 ringer, it still tells a story about this nation's history, and deserves to be recovered and recorded with respect. It's not about the stuff - the stuff is a connection to the people and the experiences. Coinshooting, which I also enjoy, is one thing. A merc is a merc. But I think of detecting historical sites differently.
Fourth, is discretion. I need to get the feeling I can trust you with my sites. That means not telling folks where we're detecting, not bringing anyone else out there without me, and whoever is the "primary finder" of the site ought to be present every time you hunt the site unless explicitly stated. That kind of trust takes a while to develop. It's about respect.
Finally, there are just some folks who I don't get on well with, morally and philosophically. They won't be invited out with me, ever, and I'll avoid detecting with them whenever possible. And if you're likely to invite those sort of people, or you knowingly interact with them on a regular basis, you won't be coming to my sites. I have to draw my line somewhere.
There's one other forum member that I've really gotten into a research/detecting groove with CW sites. There's a few more at the club that I trust or am beginning to develop a relationship with, we just haven't done quite as much together. I very rarely go out detecting alone anymore, and I much prefer hunting with friends. But those sites are very special to me, and I am protective of them. Some people may think me miserly for not sharing all my sites all the time. But I've seen good sites raided by unscrupulous detectorists in the past, so I don't feel it is unwarranted to be careful. I just wanted to explain my feeling on the subject, and provide a counter-point to the share-everything folks.
That said, if you want to detect with me, I'm always willing to give it a shot. Case in point my own farm - I'm super protective of detecting my own land, and I've gotten a lot of great things out of that ground over the years. But *every* single person who has asked me to detect it has been allowed to. I'm not unreasonable, just cautious