Forget Photoshop for ads, unless you are going to post them on the web only. If your ads are for print and contain text, you should be using a vector-based publishing program such as InDesign (rules!), Quack XPress, Illustrator, etc. You would set your type and then place your grayscale or CMYK tif images.
Problem is, text needs to RIP at 1200dpi minimum, so your file size in PS would be huge. Vector programs keep the sizes small, quality high, and are much easier to edit in.