Before I applied for an ER position, I worked on a tele floor, did urgent care, then worked on a progressive cardiac unit. This was all in a time span of about 3-4 years - the reason why I jumped around was because my husband and I relocated to another state in the middle of it all.
When I applied to the ER, I think the manager was very impressed with my cardiac knowledge, and liked the fact that I already had ACLS. Also, having a good working knowledge of several different drips (NTG, Lido, Amio, Cardizem, Heparin, etc.) also helped.
Although I don't think working on a med/surg floor should be absolutely required before specializing, I truly believe it can only help you and make you much more marketable if you do decide to change specialties. I have to thank the experience I received on the tele floor for providing me with a huge advantage when I transitioned to the ER. I was very competent at recognizing rhythms, handling chest pain patients, placing foleys, operating TC pacers, code situations, starting and titrating drips, etc. The ER can be so fast-paced that already being competent in such skills can only help you to focus on other important details such as focused assessments and minute changes in a patient's condition. Gaining experience on a med/surg/tele floor will also help you to develop great time-management skills, as well as helping to improve critical thinking skills while not having to constantly deal with such a fast-pace and quick patient turnover (not that med/surg/tele can't be fast-paced, but at least there is usually some down time most shifts when compared to how the ER has been lately). I also think that learning how the floors function first before landing an ER job can be an asset because it provides you with a better understanding of why a floor RN can't accept your ER admission right away; and helps you to understand what factors of the report you give are most important to the floor nurses and why.