Most people use deionised water or distilled water. RO water is an overkill if you were to get an RO unit. The thing about avoiding tapwater is that it might have certain chemicals which you might want to avoid like heavy metals, silicates, etc. Heavy metals can be toxic to sensitive invertebrates and silicates usually cause diatom bloom. This makes your tank glass brown very fast.
If you can live with diatoms and don't keep sensitive invertebrates by all means just stick to tapwater. I use tapwater all along. Then again different parts of Singapore will have different tapwater properties.