Malaysia and Indonesia custom ofificers are corrupted and may find fault with you in order to rip some money out of you. Check whether they did help you chop the entry stamp on your passport as they will sometimes "forget" to chop. If you never realise, then you will be treated as an illegal immigrant and detained until you pay a heavy fine. Also, Malaysia and Indonesia are not as safe as Singapore so be on your guard against robbery and rape when you go there.
If you take a bus to Johor, they will stop at the frontier so that you can have your passport examined. Remember to get back on this bus! The frontier is not the last stop. During my first trip to S'pore, I thought the bus line ended and walked across the causeway.
The SBS 170 bus starts in Queen Street about 300 m or so from Suntec City, but it may be quicker to catch the bus at the Kranji MRT (NS7).
If you want to see Johor town, get off at the Malaysian checkpoint and walk. If you re-board the bus, the next stop is very far away (more than 5 km or 3 miles away) at the Larkin bus terminal.
For tourist who want several passport stamps, a short trip to JB will easily result in 4 stamps. However, if you are short of time, this may not be a good use of time compared to staying and enjoying Singapore.