Stamps are conventionally affixed on the top right-hand corner of the envelope, and this has sort of become an international standard. When we send a letter, it is somewhat natural for us to stick it on the top right-hand corner, and not on any other corner or arbitrary position on the envelope.
In Singapore, mail items dropped into the post box before 1900 hours within the Central Business District and before 1700 hours elsewhere are delivered on the next working day. After the mail arrives at the Singapore Post Centre, it will be passed through a Culler-Facer-Canceller which tries to locate the stamp or stamps affixed on the envelope. It will then print a postmark on the envelope with the date of processing, which is likely to be on the day of dropping it into the mailbox. Each envelope is marked with a unique orange barcode at the back. An Optical Character Reader will identify the postal district as written on the envelope.
The Barcode Sorting machine will match the address and print another orange barcode on the front of the envelope, which represents the postal code of the address. If you check your mail items, you will notice that the front barcode is always the same while the back barcode is always unique. Finally, the Delivery Barcode Sorting machine sorts the mail according to the delivery route of every single postman in Singapore based on its postal code.
Now, it appears that the orientation of the stamp on an envelope does not matter. Over the years, I have accumulated numerous envelopes with fishes swimming upside down, just like the one shown below. The machine can well locate the stamp and cancel it nevertheless.
Since the orientation of the stamp does not affect its detection by the machine, I decided to experiment if the position of the stamp would affect its detection. To make it a fair experiment, only the position of the stamp is varied, while the size, the colour, and even the weight of the article remains the same. The latest 30-cent definitive was used instead of the 1st local definitive due to its larger dimensions.
On 6 July at 1500 hours, I posted three envelopes with stamps pasted on different corners. All three envelopes were delivered to my house the next day.
Probably, when the stamp is not detected by the Culler-Facer-Canceller, it has to be cancelled by hand. The absence of barcodes on either side of the envelope suggests that such items are sorted by hand. On the two envelopes with stamps pasted in non-standard locations, there are no barcodes. However, it is still delivered to my house on the same day as the envelope with a stamp in the standard location.
Therefore, it does not really matter where you paste the postage stamp on the envelope, it will still get delivered along with the rest of the mail posted on the same day. So, feel free to stick your stamp on a random spot on the envelope, but it will certainly not look professional!