Whether you're putting together a series of layouts for a client, or for a group of coworkers, be sure to create images that look like finished website pages. You may have a mind that can imagine creative possibilities and fill in gaps in patterns, but there is usually someone in your client group who can't think in those terms. You may have done your homework: studying the competition, analyzing traffic patterns, conducting focus groups, but if you present a design with "placeholder photos," there is usually always one person in the group who can't admire or accept any part of your work because "that person in the photo doesn't work here." You will enhance your chances for success if your early drafts contain photos that might actually get used in the final project. Attending to such details will also enhance your reputation as a top-level designer, and it will create work for a photographer ;-)
Exceptions to this rule: many smaller clients who don't have staff members or brick-and-mortar assets will be happy to see your selection of stock photos.