For those of you considering CPF top-ups to take advantage of tax relief -- and you should! -- here are some "rules of thumb":
1. When you're younger, your earnings are usually lower, and you're more likely to have room below your CPF Annual Limit ($37,740/year). (Or you might be one of the few newly minted Singapore Permanent Residents, just starting out with CPF.) You'll also have at least MediShield Life and, probably, Integrated Shield premiums to pay. Therefore, use your first dollars to make a voluntary contribution to your Medisave Account. Zoom it up as quickly as you can, provided you are adequately insured and have a sufficient emergency reserve fund (in Singapore Savings Bonds, preferably). There is no $7K tax relief limit here. There's only the aggregate $80,000/year tax relief limit. If, for example, you're able to top up your Medisave Account by $15,000 -- if you've got that much room below your CPF Annual Limit and can afford that hefty top up -- then you could get $15,000 of tax relief.
Once your Medisave Account hits the Basic Healthcare Sum, compulsory and "three account" voluntary CPF contributions will redirect Medisave dollars into your Special Account (assuming you're below age 55).
Newly minted PRs that can afford it should take advantage of the fact that, during the first two years, they will have LOTS of room below the CPF Annual Limit due to lower compulsory contribution rates. So, if they can afford it, they should zoom up their Medisave Accounts quickly -- for example, with a Basic Healthcare Sum of $52,000 (2017) it'd be nice if they could voluntarily push, say, $30K in immediately (right after picking up a blue card) and the balance in January the next calendar year.
2. If you've got some more top up dollars, then focus on your Special Account top-ups. Yes, you can do both MA and SA, but MA is usually best to do first because of the way overflows work, because you may not always have room below the CPF Annual Limit, and because MA dollars may immediately be useful for qualified insurance premiums and medical expenses. Up to $7K/year of SA top-ups qualifies for tax relief, for you.
3. If you've got some more dollars, then focus on topping up a qualified relative's Special Account, such as your spouse's SA. You may be eligible for further tax relief of up to $7K, for total SA top-up tax relief of up to $14K.
Some people would recommend reversing #3 and #2, or at least putting them on equal footing. I think there's strong merit in that idea, but it depends on your circumstances. If you're topping up a non-working spouse's Special Account, for example, then what you're really doing is boosting that spouse's future lifetime annuity payout. There are good insurance-like benefits to doing that.