Top up when you need funds to buy something. Not sure how you put funds into US account last time, like whether you buy physical cards from local game store (at marked up price), but nowadays you can buy US psn cards digitally online. You pay the same amount as the face value (so USD10 card you pay USD10, or even less when there's sale). You can buy from Amazon US or pcgamesupply, and pay using SG cc/paypal. And you get the code instantly. So it is no hassle to top up US account wallet.
For SG account, you pay using SG cc/paypal directly, no need to top up either (but if you want, can buy SG psn card physically from Cheers stores island-wide). If you pay directly, there is a minimum SGD10 per transaction limit (so if you have $0 balance and buy something $1, they charge $10 and your wallet is left with $9; if you buy something $11, they charge $11 exactly and wallet balance is $0). (A 1% surcharge may be incurred when paying via cc.)
Either way, don't have to worry about topping up the wallet yet. Do so when you have something to buy, especially during sales. Keep an eye on the Game Deals thread for news.
And, no, Asia is usually not cheapest. US, Canada, and often also UK, have regular (weekly) sales that can be much cheaper. Whenever you want to buy something, look up this site psprices.com for price in each region and the price history.
btw, if you use US account to buy something from US store, you can typically use SG account to play it. Most games are region free (though some rare case, like microtransactions, may be locked to the purchasing account).
