Should recommenders be from the same discipline as the program one's applying to?
To point out one pitfall in kmdouglass's answer, there is one situation in which getting letters from out-of-discipline people can affect an applicant's chances: if the letter writers do not directly support the candidate's application to the specific discipline. In other words, if you're a mathematician applying to physics programs, your math professors should be explaining why you will be a great physics graduate student, not a great math graduate student. (Or why you will be a great graduate student in any department.) "Dissonance" between the letters of recommendation and where you're applying could make some reviewers question if you're seriously interested in applying to physics departments, which could hurt your chances (albeit perhaps only slightly).