This is so important, in order to gauge a company's interest in you. If they tell you what time you're going to be interviewing; it's a cattle call - your chances of getting the job are slim. You're merely one of many. If you can pick the time, there probably aren't so many of you competing for the same job, so your chances are a little better.
Of course you'll have to travel to the client site - there's no negotiation on that. But insist on an early morning or late afternoon slot - even if you aren't working. It gives the impression that you are, or at the very least, that you're in demand. Make them wait as long as possible to see you if you can. It drove one recruiting company nuts that I put them off for a week for a phone screen with their client. Sure enough, I got the second interview.
Just look for the red flags that the hiring company or recruiter is trying to exert too much control. If they have an attitude problem on the phone, demand an extensive series of tests, or ask to see a pay stub history - gracefully exit the process with them as quickly as possible. They should be pursuing you. If they're not calling to pin you down about when you can interview, if you want the offer, or if you still want the job, then maybe you're not their first choice, or maybe they won't treat you well once you finally join them.
During the interviews, whether it's a phone screen or an in-person session - have a good attitude. That's key. Don't badmouth your soon-to-be-former colleagues or bosses. The IT world is small, especially here in Denver, and everyone is a friend of a friend. It's more like three degrees of separation than six. Word will get back to people. Client companies love to unload everything on recruiters... and those recruiters have long memories.