Money data type in MySQL?

[David Garamond]

What do people recommend for storing money amounts? I've seen people use 
NUMERIC(18,3) and other use NUMERIC(18,4). Which one is more appropriate 
and why? This is considering various existing currencies, some having 
low rates (like IDR, in which you can have large amount up to hundreds 
of trillions) and some high rates (like USD, in which you can have small 
amount like 0.1 cent). Are there places/industries which involve values 
lower than 0.1 cent?

And what about 'factor' field in currency conversion table? Should I use 
FLOAT, or DOUBLE (do we need 15-16 digit precision?) or NUMERIC (exact 
numbers). The factor should range between 1E-3 (e.g. converting IDR to 
USD) to 1E4 (e.g. converting IDR to pounds/euros).

-- 
dave