Sample every variation of active lesion that you can find, within the limits of humane practice. If you think certain lesions are perhaps a different disease, submit them in a separate container. Average case should yield 3-4 biopsies. Randomly selected biopsies from chronic disease, biopsies taken within 10 days of the end of steroid therapy, expectation that anyone can prove the presence of allergic skin disease. Needle aspiration, core, incision, excision.
Excision is always the most suitable for diagnosis and especially for prognosis.
One possible exception is investigation of lymph node enlargement, because fine needle aspiration is probably as good as any technique for confirming suspected malignant lymphoma. Nasal swabs, nasal flushes, traumatic flushes, blind biopsies, endoscopically-guided biopsies. Everybody gets lucky once in a while, but only endoscopically-guided tissue biopsies routinely provide useful results. Endoscopic mucosal biopsies, exploratory surgery, rigid proctoscopy. Endoscopy provides shallow biopsies that are difficult to interpret with respect to malignancy.