I think it's fairly common to take some time out before starting a masters, and to be honest, I don't think anyone cares that much if you do or not - but life/real world experience can be a bonus when applying for masters degrees and jobs in the future.
There's not really such a thing as a 'gap year' between undergrad and postgrad degrees like there is for 18-year-olds between high school and university, but there are a wide variety of ages of people doing masters degrees and many are mature students.
I took a year out between my undergrad at Exeter and my masters at Bristol, and my masters course consisted of students aged between 22 and 51 - we had a few students who came straight from undergrad, but several of us had taken at least 1 year out before starting the masters programme - I was 23 when I started, and we also had one 24-year-old, two 35-year-olds and one 51-year-old as well.
My Exeter degree was an MPhys, which meant it was a 4-year degree anyway, giving a masters at the end. I didn't know what I wanted to do afterwards, so I applied for a couple of jobs and then got my vacation job back at Boots while I also applied for PhDs and MSc programmes. I ended up working at Boots for a year and living with my parents, so I could save up enough money for my masters degree.
In the end, I think my extra retail experience (between age 15 and 27) worked in my favour for getting my current graduate job, as although I work in a scientific area, my job consists of a lot of customer interaction and public speaking.
Interestingly enough, the two jobs I applied for in 2005, as I was finishing my undergrad degree, were the Met Office and the British Antarctic Survey (BAS). I didn't get either job at the time (hence the masters), but I started working for the Met Office in 2010 and right now I'm living in Antarctica, on a 3-month secondment with the British Antarctic Survey (I've been here 10 days so far).