David,
They've changed some of the info in that link. I upload all my files at 300 dpi and set the long side for the largest file I plan to sell. I also set the jpeg quality at 70, which iirc is what SmugMug's info used to recommend.
Locating the link for you gave me a reason to review it. I'll be changing my jpeg output quality to 80, but I'll still only upload at 300 dpi. I always set a 5 day window on purchases although I've not had to delay on any order.
If it becomes necessary to upload a larger file to ensure any larger prints ordered have great print quality, I'll upload the individual file as needed.
My preference for fulfilling large images is to print them myself, hence the large format printer that resides at my place.
I've tried using the new SmugMug Plugin that is now built into LR3 (3.2) and have found that it doesn't have the control of other tools already available, particularly for the conversion to jpeg. As a result I'll continue to convert to jpeg first, then upload to SmugMug using "send to smugmug" by Omar Shahine.
Previously I used a LR plug-in that used imagemagik as the engine to "mogrify" files into jpegs. It had a lot of control over the final output, and I'll be looking into it's compatibility with LR3.2 later this week. You learn more about it at
http://timothyarmes.com/lr2mogrify.php