Is it wise to max out spouse's SA account first?

I second Yixiong suggestion. It's best to have $60,000 combined CPF for the 5% interest (the additional 1% goes into your SA).

Other than that, it really depends on your investment appetite, whether you are savvy in investment or you wish to gain the guaranteed 4% from SA / MA (special and medisave account)

And the Tax Relief for some of the following:
1) Voluntary contribution (for self-employed, tax relief)
2) Retirement Sum top up of 7k max (for money not above Full retirement sum (FRS).)

Do Consider index investing (though always invest in what you know to minimise risk).

The only advantage is if any of your CPF has not hit $60k combined balance. For the first $60k in your CPF combined balance (with only up to $20k max from OA), the monies will earn an extra 1%. Anything above, both your CPF monies earns the same interest.

You will receive tax relief for the topped up amount into your special account, Up to $7k a year.

Eg, your taxable income is $40k, for every dollar u earn above 40k is taxed at 7%. Therefore, if u topped up $1000 into your special account, u get $1000 tax relief, u saved 7% of $1000 = $70 tax money. Effectively have gotten 7% return on your money,

So depending on you and your wife's taxable income, u can decide how to best topup to maximise the tax relief.

Do add me on FB if u wish to discuss further. =)