Buying indexes - monthly allocation

[banders]

I'm sure this has been addressed elsewhere, but if I’m manually buying funds each month and I’ve decided on, say, an 80% stock/20% bond allocation, should I top up each index every month so as to bring it back to 80/20, or should I just pay 80%/20% of my monthly payments into the stock/bond indexes, ignore current balances and simply rebalance once a year?

[RetireJapan]

Either works, but personally I would adjust each month to get to the desired allocation. Buying the lower asset is usually better than buying and selling to rebalance (time, taxes, etc.).

Once a year if your portfolio is really out of whack you could then rebalance as well.

[banders]

I forgot about taxes on selling! The bulk will be in taxable accounts. Then if the amount I need to add to bring the balance back becomes more than my monthly payments then I can sell and rebalance once year. I see. Many thanks!

[jcherni]

I have all automated, with a fixed amount I invest every month.
From this amount, 80% go to stocks and 20% to bonds.

I only rebalance once a year, when I receive my winter bonus (I divide it so that my portfolio goes back to 80/20).
So far it has been enough to rebalance without even need to sell.

But I'm sure eventually I'll need to sell some to rebalance at the end of the year.