When my broker shows the market value of my whole account, they round
each account and add everything up, i.e. 10.12 + 20.12 = 30.24.

However, ledger keeps all the precision, calculates 10.123 + 20.123 =
30.246 and shows 30.25.

Is there a way so I can tell ledger - ideally on the command line - to
limit precision to 2 digits, so I'll get the same result as what my
broker shows?

Test case:

D 1000.00 EUR

2012-01-01 *
   A                        1 AAA @@ 10.00 EUR
   A                        1 BBB @@ 20.00 EUR
   C

P 2012-07-01 AAA 10.123 EUR
P 2012-07-01 BBB 20.123 EUR

$ ledger -f a bal -V A
           30.25 EUR  A

The problem is that you can rewrite the individual posting amounts (too fine),
and you can rewrite the final account totals (too coarse), but you can't
rewrite the individual account totals in between.