When uploading an image to say mpix or another web printing service, does it matter, in terms of quality, to upload in tiff or jpeg?
You really have to ask that?
OK, I'll give you a serious answer. It depends on the source.
You would think that TIFF is the answer. Well, it is, assuming the original source was some sort of lossless format and has never been destructively compressed. So, your source is a lossless format photo, or an original digital image produced in Photoshop, Illustrator, etc.
You certainly would not want to convert that to JPEG, because you will lose quality.
If the original source is JPEG, though, that might be a different matter. You have to realize that if your original source is JPEG, it's already tainted. You just have to do the best you can with it. If you need to resize or adjust it in any way, you are going to lose some quality. And it had lowered quality from the git-go.
If you know what you are doing, do whatever editing/adjusting, and ave as a TIFF. You start with a degraded image, manipulate it, but why degrade it further?
On the other hand, if you are going to use the original as-is, without any changes, then you might be better off submitting the original JPEG. The printer probably has better conversion/filtering tools than you, and they know what works. The printer is used to people throwing nasty JPEGs at them, and they probably have some automated workflow set-up to to the best they can with it.