Interviewing is a skill, and negotiation doesn't come naturally to most of us.
I believe the most ideal time to interview is while you're fully employed:
Eventually it may be the case that you find yourself interviewing while unemployed, desperate, hungry, and if that's the time you go out to do some practice interviews you won't be in such a good position to negotiate with them.
Interviewing is tough tough work, and it's draining and expensive (in time, and even sometimes money) for you and the company, but over time it can help you present yourself well and make better deals.
If you have the opportunity to interview, I think it's probably worth the exercise!