Of the three choices available to married couples joint/aggregated assessment is normally the most beneficial for most couples. Where both couples are maximising the use of their credits/allowances there may be no immediate benefit though but there should be no harm in switching to it just in case circurmstances change in the future (e.g. one spouse gives up work or becomes unemployed etc.). Check out the [broken link removed] and perhaps crunch the numbers using [broken link removed] to see how joint assessment might affect your particular situation.