に越したことはない【にこしたことはない】

に越したことはない means "nothing beats ~", "it's best if ~": the thing before it is the ideal, though not strictly necessary. It goes after the dictionary form or ない-form of a verb and after an い-adjective. A noun or a な-adjective takes it directly, with no な or だ, or with である in formal writing.

• × 体は丈夫なに越したことはない → 体は丈夫に越したことはない

The literal sense is "nothing surpasses ~". It states what is ideal as general common sense, which is why できれば ("if possible") and もちろん ("of course") so often come with it.

Conceding with が or けど

The second half of the sentence often turns: X is ideal, but it isn't essential. に越したことはない followed by が or けど sets that up.

に越したことはない or ほうがいい

ほうがいい is everyday advice about a particular case: "you should". に越したことはない is the general rule, and leaves room for not following it. It is common in written Japanese and in measured speech; with friends, ほうがいい is the usual choice.