Skyteam, I don't think you will encounter problems during the daytime hours if you stick to the better areas of Managua. One way to avoid potential problems is to use a driver recommended by the hotel (as opposed to flagging down a taxi on the street). My own view (untested) is that I'll be fine so long as I exercise the same amount of caution I would exercise in, say, NYC or LA.

A good resource for reports from travelers in the region is the Lonely Planet Thorntree forum:

http://thorntree.lonelyplanet.com/index

Scroll down to the Nica section. Enjoy your trip!