I never consider interviewing to be a waste of time. At the very least I get some very valuable practice out of it, learn more about what's out there, and meet some interesting people.

Even if they wind up not hiring you because they find out you're too expensive for their current budget, their budget might increase in the future and they might try to hire you again. Or the people who you made a favorable-enough impression to want to hire you might look to hire you when they're working for some other employer with a bigger budget. If you disqualify yourself by naming too large a number at the outset you'll never get the chance to make that great impression.

Finally, not everything's always about money. If you wind up at the point where they want to hire you and finally you start talking numbers, if their best salary offer is below what you're looking for there might be able to compensate you in some other ways that you could both find acceptable -- that is the essence of negotiation. Here once again, if you had disqualified yourself by naming too high a number at the outset, you'd never get the chance to negotiate.