Yes if you are booking online direct at the singaporeair.com website. A "choose seats" button will appear on the shopping cart review page immediately prior to payment.

This is good because you dont have to pay and commit first, for instance if you are looking for preferred seats etc and are flexible in which flights you can take.

Note that if the flight is already heavily booked you might get the generic message "Seats available for advance seat selection for this flight segment have been fully taken up. You may select a generic preference or select once this flight is open for check-in". this will then allow you to state a window/aisle preference and then basically see what you can get a online check in.

I believe that travel agent bookings are also able to select seats in advance but I have no experience in how this works...