My own opinion of SL is that the people there are generally pretty nice, not disposed towards violence or robbery, and it is overall one of the better countries in Africa. This opinion is based on the many trips I have made there for work, and lots of discussion with friends and co-workers who have lived or presently live in SL.
I don't think you need to "avoid" SL - if anything, it's one of the least stressful countries to be in, although the country doesn't have much going for it - it's not exactly a tourist destination.
You need to be mindful of the fact that the people there are generally very poor, even though they are honest and pleasant by nature. I don't think you need to go to the trouble of travelling in a group, simply use your own common sense - travel by daylight, avoid looking too wealthy, if you see an area you think you should stay out of, stay out of it, etc. In other words, take the same precautions you would if you were in an unfamiliar and poor area of England or America.
The real question you need to ask yourself is where you plan to go after SL. I would NOT recommend going into Liberia - that country has serious problems that are getting worse, not better (at least SL is improving, not going downhill). No way in heck I would ever drive through Liberia, even if it was in a tank. As you continue to progress east along the Gulf of Guinea, things generally get worse, so far as personal security is concerned - Ivory Coast has troubles, and don't even think of going to Nigeria. Ghana is about the only nice place east of Sierra Leone, at least, until you reach the Maldives.