Nice jeans. But should you really wear them to the opera?

They've been part of the American "uniform" for years, worn to casual restaurants, house parties, and some workplaces.

But as jeans become more expensive, they are also becoming more ubiquitous, showing up everywhere from Midwestern churches to Broadway shows. Nothing is off limits, it seems. Or is it? Among those who buy high-priced, designer denim or who simply don jeans frequently - there's debate about where it's appropriate to wear them.

The tug of war over jeans etiquette is particularly prevalent in New York City. Here, people tend to be more creative about their appearance, and are often more demanding about how fashion-conscious people should look, says Dannielle Romano, editor at large for DailyCandy.com, a fashion and trends website.

Many 20- and 30-somethings here have theater backgrounds, for example, and often say it's inappropriate to wear jeans to theater and other cultural performances out of respect for the performers and the surroundings (even though the venues themselves have no official dress codes).

"I am all in favor of the current denim revolution that we are having, but I do feel that there are times when jeans should be left at home," says Lisa Kerson, a jewelry designer in her early 30s, whose parents insisted that she look nice when going to a play or traveling on a plane. "I still get bothered when I see people wearing jeans to the theater, ballet, opera, etc.," she says in an e-mail.

Melissa Popiel also prefers not to see denim at the theater, or at an engagement party. To her, jeans are OK for a house party or a casual dinner, but not for traditionally dressy places. "I don't like going to cocktail parties and seeing people in jeans," says the advertising executive, who's in her late 20s.

Ms. Popiel estimates she owns about 15 to 20 pairs, including premium brands, and has paid as much as $200 for a pair.

Many others are also paying big bucks for their jeans - from $150 to $1,000 or more per pair. Celebrities, in particular, are making jeans their garb of choice for appearances on talk shows and at some red-carpet events.

That, say fashion experts, sets the tone for the masses, who are encouraged by features like one in the Jan. 24 edition of Us magazine, "Hollywood's 10 Hottest Jeans," complete with suggestions for buying "premium" denim ($140 or more).

The concept of designer jeans is not new, however. They were also hot in the 1970s and 80s.

Etiquette experts offer few hard and fast rules about jeans, but among them are the obvious: Leave them in the closet when you're attending a wedding, or if your workplace bans them.

"A lot of it has to do with the appropriateness of the kind of jean you're wearing," says Peter Post, grandson of manners maven Emily Post and author of the book "Essential Manners for Men."

It comes down to determining if the jeans are for fashion or work. A pair that you do yard work in, for example, are "probably not appropriate to be wearing to a restaurant that night," he explains.

Mr. Post has seen men show up in quality restaurants wearing denim, which doesn't bother him as much as how sloppy their appearance sometimes is.He recalls seeing a man dressed in a T-shirt and old rumpled jeans. "He hadn't taken any care to step it up just a notch, to say to the woman he was with, 'You know, you're really important to me. I want to look good. I want you to look at me and be proud of me,' " he says.

Dark denim is making it easier for men to comfortably wear jeans in the evenings, especially since black jeans are no longer "in." But no matter how hip a certain style may be, some places are still off-limits.

"I probably won't wear them to a funeral," says Robert Smith, a 30- something businessman in Rockton, Ill. But in the past few years he's started wearing them everywhere else - to church and to most work-related functions.

The good news for jeans devotees is that standards for judging people on their appearance are loosening a bit - at least among women under 40. A recent study by Cotton Incorporated indicates that Generation X-age women (26 to 39) are less concerned about first impressions when it comes to dressing than they were 10 years ago, and more often are taking the approach that "you can't judge a book by its cover." The reverse was true for women boomer-age and older.

Alice Harris, author of the book "The Blue Jean," attributes the rise of jeans to casual Fridays in workplaces, which shifted the way people viewed dressing.

"We've actually gone back to a much simpler way of looking at it," suggests Post of the changing attitudes. It's not that certain materials, like denim, are bad. "It's what you've done with that material."