When does it make sense to top up my parents RA, instead of giving them monthly allowance?

For me it's 3 reasons:
1. My parents are still working and do not need the money.
2. My parents are a bit of a spendthrift, so having the amount slowly distributed when they retire would help with them not spending it all at one go.
3. I get tax relief of up to 7k

If their RA account is less than $30,000 I will top up their RA account so that they can earn the 6% that is provided.

6% is a good rate worth while toping up.

Aother note is that money credited to RA cannot be withdraw as a lump sum, your parents are getting it monthly payout. In the long run its good as meanwhile they are earning 4% from whatever is left in the CPF that is not being paid out.

However in the short run, what they are receiving from cPF may be less than what you can directly pass to them.

Check with them if they prefer to have it now. Sometimes they may feel they have very little time left to enjoy the spend in the next decade ahead so they prefer to go enjoy a little bit before they are too weak in energy.