Note: This will only affect 386/486SX/486SLC owners - other machines will have one built into the CPU.
In general this will not cause any problems, but there are circumstances where you will take a hit, either in performance or accuracy of the math emulation code (see the section on FP emulation). In particular, drawing arcs in X will be VERY slow. It is highly recommended that you buy a math co-processor; it is well worth it.
Note: Some math co-processors are better than others. It pains us to say it, but nobody ever got fired for buying Intel. Unless you are sure it works with FreeBSD, beware of clones.