Snap tools to guide lines

I am often masking off product images to white out the background. The best results I have is unfortunately manually painting a mask around the areas of photos. I frequently use guide lines to draw straight lines and the constrain to keep things horizontal and vertical. But what happens when you use a custom angle on a guilde line? Now you can't constrain. I have to eyeball my cursor along the guide line and that takes tons of time. I use a trackball mouse and are very good at it, but if the cursor for whatever tool you are using would snap to the guide line, I can mask off, or color or clone etc. much quicker and with more accuracy.