We currently have a central computer room with several distribution closets (11) over 3 floors. We have fibre backbone to some of the rooms (18 multim

Most electronics are available for both multimode and singlemode fiber, but today singlemode is usually for long distances. However, singlemode fiber is so inexpensive, that adding some singlemode fiber to every cable you install can make sense and someday you may need its bandwidth. (Remember 10 GB Ethernet is coming!)
I have been recommending installing hybrid (MM+SM) cables for ten years as a hedge on future applications. Don't terminate it now, but have it available for future networks that may require it. Instead of a 18 fiber MM cable, price a 24 fiber cable with 18 MM and 6 SM. You will find the cost differential quite small and if it saves installing another cable in the future, it will have enormous payback!