Shouldn't it be:
It's rather trivial in this case; nothing will be harmed by assigning three floats to themselves. Past attempting to maintain the invariants of the class, there is little other reason to check to see if you are assigning to yourself for a class that contains nothing but primitive types.
If you don't need to explicitly test for self-assignment, for example, if your code works correctly (even if slowly) in the case of self-assignment, then do not put an if test in your assignment operator just to make the self-assignment case fast. The reason is simple: self-assignment is almost always rare, so it merely needs to be correct - it does not need to be efficient. Adding the unnecessary if statement would make a rare case faster by adding an extra conditional-branch to the normal case, punishing the many to benefit the few.
In other words, don't stick one in just to do it. Do it for a provable reason that isn't speed.