I think it's fine. After all, most jobs are just jobs -- not careers. When you get a job, you're not promising to spend the rest of your life there.
That's so true these days. Way back when, it was a different expectation. People took jobs they expected to stay in. We have a LOT of people that came to the library as their first job 25-30 years ago and have never left. These are the people that will be retiring within the next 2 years or so.
I believe it's ok to job search while you're employed, if there is a reason to job search. I always caution against blindly job hopping though, just to be changing jobs - it catches up with you after awhile when recruiters look at your resume - unless it shows a progression of increasing responsibility.
When I hire people these days, I don't expect them to stay with me forever. I ask that they give me their best and that they learn something while they are in their job so they can take the experience with them where ever they go.
I hired a administrative asst. as a payroll clerk, no payroll experience at all. I taught her HR basics and payroll, when she left after 3 years she was able to get a good job in HR. I was happy to watch her grow in the position and extremely pleased when she was able to get a good job when she left. We had talked about her leaving one day. I allowed her to interview during the workday so she didn't have to sneak and call out, it worked out well.