My understanding is that pems can not have white above the point of elbow on their legs or on their bodies, except for a white collar or white blaze.
From the PWC Standard:
Color-The outer coat is to be of self colors in red, sable, fawn, black and tan with or without white markings. White is acceptable on legs, chest, neck (either in part or as a collar). muzzle, underparts and as a narrow blaze on head. Very Serious Faults: Whitelies-Body color white, with red or dark markings. Bluies-Colored portions of the coat have a distinct bluish or smoky cast. This coloring is associated with extremely light or blue eyes, liver or gray eye rims, nose and lip pigment. Mismarks -Self colors with any area of white on the back between withers and tail, on sides between elbows and back of hindquarters, or on ears. Black with white markings and no tan present.
PWCCA Here's another link that has a great set of pictures for examples.
Cardigans are different and can have white in those areas, but not pembrokes.
If it's preferred, I can't say. As per another discussion about Cardigans and mismarks, the more white you have, the higher chance of breeding mismarks which of course per the standard is an disqualifying fault and for the most part not a practice which a responsible breeder would breed for purposely.