Welcome to FT!
Can I assume that you've used the usual resources (Kayak, ITA Matrix, etc) that show you a calendar of fares around the days you wish to travel? You've tried playing around with different airports NRT/HND, SFO/LAX/SEA I assume...In that case I would just take the fare that you see. SQ is great, and whilst you may find a cheaper fare a few weeks before departure, it's just as likely that you won't, and that everything will be a lot more expensive. Either way, saving $50-$75 isn't worth the peace of mind.