I'm approaching it from two perspectives. For my own work, I buy standard frames and then customize the mat opening. But printing is always decided first so I an making a final crop and printing to a specific size. I inventory a supply of my most frequently used mats for standard sizes because it's so much cheaper to buy in bulk through FrameUSA or Frame Destination compared to having a single mat cut locally.
I sell a lot of event images, and my online fulfillment service provides a range of popular size options. But when I post images for review after an event, I'm posting a good cropped image that customers will buy - not the entire frame. If I post an 11x14 crop because it looks good, I may end up selling a print that is smaller or larger - and certainly a different proportion. So in some cases, I have to go back to the original, recrop, and substitute the new version to fulfill the order. It's impossible to have a one size fits all approach as even a 3:2 crop may not fit an order for an 8x10 print. On a few occasions I did not have room in the original file for the desired crop, so I ended up adding canvas and stretching the image to fill the space. Content Aware Fill in Photoshop does wonders and can handle filling canvas if needed.
The other thing I encounter when I have to recrop is I may have cloning or other local edits that are outside the original crop but within the crop I ultimately need. I always want to share or post my best work, but if a client wants something different, I need to make that work.