Okay first up, here's a link as to more IRAS guidelines as to what they actually mean (and it appears my initial interpretation is correct):
http://www.iras.gov.sg/ESVPortal/gst/gst_c.7.3_price+display+for+goods+and+services.asp
Now, on to whether these guides are supported in law.
I've included an extract from the Goods and Services Tax (General) Regulations which supports the above guidelines as follows:
Price to include tax
77. (1) Where any taxable person displays, advertises, publishes or quotes in any manner the price of any supply of goods or services he makes or intends to make, such price shall include the tax that is chargeable on the supply under the Act unless the Comptroller approves otherwise under regulation 78.
(2) Paragraph (1) shall not apply to any display, advertisement, publication or quotation which is intended solely for the purpose of making the supply to a taxable person and which is in a form not ordinarily available for distribution to the public.
Exemption
78. (1) Any taxable person who desires to be exempted from the requirement under regulation 77, shall apply to the Comptroller in such form and manner as the Comptroller may determine.
(2) The Comptroller may, if he thinks fit, approve in writing the application, subject to such conditions as he may impose.
(3) The Comptroller may, for reasonable cause, at any time revoke any approval granted under paragraph (2). Hope the above helps explain a bit more.