Sure, creating local objects on the stack isn't an issue (it's an essential part of implementing the language, and the programmer doesn't have any control over it, so it's GOT to be done efficiently).
But explicitly or implicitly creating objects on the heap can get very expensive if you overdose on it.
I agree about using standard library container objects (though they don't add much functionality here) - but the same memory access issues apply either way, even if the containers make them less obviously visible.
"Premature optimisation is the root of all evil" is also a good motto - but it's a fair bet that something like an equation solving routine will be worth optimising. Letting a container class expand itself incrementally to the right size, when you already know what the right size is, just being lazy IMO.