Bencoolen Street is quite a long stretch from the Rochor area to the museum/university district. The Rocher end fringes the Waterloo/Sim Lim shopping area. The lower end is mostly hotels and private residences.
It is considered a major road, and so it will still relatively be busy even late into the night, esp on weekdays. Good news though, few in Singapore, if any at all, would consider it a dangerous area. Apart from a popular cafe at the heart of it, there are no also pubs and so on. As long as you take the usual precautions when walking about late in the evening in any city, you will be fine. Frankly, the area is as safe as practically every other part of Singapore after dark.