tips are not expected in Singapore- we even had the funny situation of a taxi driver shouting us back as he thought we had made a mistake. Funnily enough I can't remember what the case was in KL. I do know that taxi drivers were a nightmare and would turn away any journey they didn't think was long enough( and our journeys were always not within walking distance) and so we had to offer more than the standard fare as we got fed up in the end.

In KL I usually tipped unless service was really bad, but as food and taxis are so cheap there even a good size tip costs next to nothing when converted back into pounds. As previously stated taxis in KL can be of vary quality it depends on the mood of the driver (they only go where they want to and will turn down fares if they can't be bothered) sometimes they will try to overcharge you with a dodgy meter (but because of the exchange rate it usually results in paying an extra £ or 2). To combat this I use taxis provided by the hotel (cost a little bit more but are of better quality and they don't overcharge) or if that is not possible I will negoitiate a fee (just ask how much to ?) before I go anywhere (they will probably charge 10RM for a short journey (£2) but it stops the chance of a larger rip off.
The biggest con to look for is the taxis from the airport do not get a taxi from the arrivals area after you come out of the security bit as they will charge up to £60 for the journey, instead either book a limo/taxi from the booking stand just as you leave the security area it will cosy about £20, or go out into the public collection area and take the lifts down to the trains and catch the express into KL £10 return and only take 40 minutes instead of 90 minutes by road (then you can get a taxi from the train station for about 10RM and there will be a booth to order it from) or catch one of the 3 tranit systems to your hotel.

We've always used a limo to/from our hotels in KL at about £11 each way...the journeys never taken us more than 50 mins and if there's more than one of you no great price difference compared to train

I have always flown the MAS flight that arrives in KLIA at around 7AM(the travel agent usually throws in free transfers) but we seem to always arrive at the outskirts of KL (near the small airport) and hit a traffic jam every time which takes ages to clear (that said I have not been since the opened the flood tunnel toll road which is supposed to knock about 20 minutes off the journey time).
I think I would use the if I was carrying very little luggage or was staying at one of the 5 star hotels that sit atop the train station.