Arbitrary precision rationals increase in storage and processing time each time you multiply them, so they open you up to DoS attacks. My order of preference:

Arbitrary precision rationals increase in storage and processing time each time you multiply them, so they open you up to DoS attacks. My order of preference: * Fixed-point arithmetic: All-around safe, though you'll probably need to take a maintenance window later if you need extra precision. * Floating point arithmetic: Sounds dangerous, but you can eliminate most of the risk using database checks. * Arbitrary-precision arithmetic: Almost any place they're used opens you up to DoS attacks; it's very difficult to detect the problem. I'd ensure my database and application server have debugging symbols installed, replace libgmp with something that alerts by generating core dumps when a timing threshold is passed, use a language where backtraces are easy to read in gdb, and ensure that all arbitrary-precision arithmetic is written against gmp. I'm not sure what I'd do when I detected the problem though: If I truncated 1 + 1/(10^10)! to 1 to resolve the issue, it's equivalent to using floating point in the first place.