A good question. I normally tell mentees to gain at least some little experience in a field (for a year or two or maybe three) because it makes a huge difference. It opens your mind and allows you to look at the world from a different angle (different from what you have learnt in undergraduate level). Nonetheless, some have been lucky enough to do a masters degree immediately after undergraduate and have gone on to do very well. Better still, in my masters class, I had colleagues who took a gap year in their undergraduate medical course to pursue masters, and they say it changed their perspective. I also know some of my undergraduate colleagues who rushed on after campus to do a masters degree that they have never used to date. Therefore, take up opportunities if and when they come! There is always an opportunity of doing even two masters.