My contractor told me that it is common to engage electrician without license for electrical works (but all work will still follow HDB regulations). If we need endorsement from Licensed Electrical Worker (LEW), there will be an additional charge. How true is this according to your experience?

Yes, it is good to have licensed electricians for the electrical work.

LEW is needed when major changes to the electrical works are carried out. Usually, when you want to add power point, add lighting points etc, you don't need LEW licensed electrician for the job.

I think for simple jobs like installation of lights, changing of switches, any person with training can do it. (My father used to change the sockets in the house, and we change bulbs in the house, right?). But I think re-wiring will require an properly trained one. 

My ID also claims no need HDB licence.

Chelsea lighting shop @ Jurong where I got my lights from says they use "experienced" technicians.