Confused About Print Bleed

I just can't seem to process this one mentally for some reason. I usually send my finished images to ProDPI for printing. Say I have a landscape or nature photo, for example that I want a 12x18 inch print of.

In Photoshop (cs6) should I be resampling the image to 12.25x18.25 inches, then sending it off?

Or should I set the image up as normal - for example 12x18 and then increase the canvas size by 1/4 inch (then fill the empty canvas with black or white)?

What does the print service need?

For standard photographic prints, you don't need much bleed.

For other specialty prints such as metals and press-printed cards, you need some bleed since parts of the product may be cut off after printing, OR the printing equipment is not precise enough to line up the edges of the medium to the edges of the image, right down to the pixel. Or maybe some other reasons I don't know.

Generally, you'll add the bleed amount to each edge. So, 0.25" bleed + 12" image + 0.25" bleed = 12.5" final Photoshop canvas width.

DON'T just add extra black or white to the Photoshop canvas. If the cutter is misaligned by 1/32" then you'll lose a tiny bit of your image on one side (nobody cares), but more importantly you'll have a 1/32" black or white stripe along the edge of the final product (UGH).

Instead, make your image 12.5" x 18.5", and make sure no important compositional elements are within 0.25" of the image edge, 'cause they're getting cut off from the final print. Better yet, leave a margin of 0.5" or even more, where you have image but not IMPORTANT image. Maybe the process is going to cut off 0.3" from one edge and 0.2" from the other edge. No guarantees of precision down to the pixel.

Similarly, if you're doing a graphic layout such as modeling/comp cards, most print services will recommend against including a colored border around the edge of the design. If you have 0.25" cut-off bleed, plus an extra 0.25" black border on your graphic layout, maybe you're going to lose 0.3" off the right side and 0.2" off the left side, and end up with a remaining border that's 0.2" on the left and 0.3" on the right. Unsightly.

Some printed products, such as gallery wraps and book covers, need WAY more bleed in order to have decent content to wrap around the sides & back. A one-inch-thick gallery wrap will need an extra 1.5" on each edge - an inch for the side of the stretcher frame, and a half-inch wrapping around the back of the stretcher frame. So for my 11x14" gallery wrap, I needed to submit an image that was on a 14"x17" Photoshop canvas, AND make sure no important content was within 1.5" of the sides... or really, within 2.0" for safety and composition.

Many print services will offer Photoshop templates, with guides or overlays to help you out.

Yip that covers it really. But honestly a small bleed will hardly be noticed unless you have critical elements on the very edge of the image.