Hello Bob - This answer is also partly in response to your earlier question on print sharpening. Also I think you may know some of this, but others may not and hopefully find this useful / stimulating . . .
Here's my practically-orientated take on this. Assuming there are three key steps to sharpening - capture sharpening to do with specific characteristics of camera, creative sharpening where you get the specific picture to look as you want it, but on on screen, and output sharpening, where you try to counteract the paper (and printer's) tendency to undo your creative optimum.
Aperture and you deal with the first two steps prior to the print dialogue. So what one should *only* be trying doing in print sharpening is counteract, in particular, ink spread on printing, especially noticeable on matt papers such as Photo Rag. So my suggestion would be to take an image with a lot of fine edge detail such as twigs or grass, sharpen it to your creatively judged optimum before reaching the print dialogue, and then print off a number of times with well spaced, systematic, choices of print sharpening. Then compare the different prints with your image on screen, (important - not while in the print dialogue) at 100% and with you some distance from the screen. You are looking for the print which you judge is the closest in sharpening appearance to your creative sharpening on screen - you should resist any temptation to do more creative-type sharpening at the print sharpening stage! You may need to iteratively home in on a particular adjustment for a particular paper. Remember the setting combination for the particular paper/(printer - if you have more than one - but the paper will have the more dramatic implications) combination.
As this is more dramatic with matt papers, I'd also suggest experimenting first with matt paper, or even just low quality uncoated photocopy paper to get a working understanding of what happens with what combinations. Then do the same with glossy coated paper, where I'd expect the adjustments to be more subtile. This print adjustment can then be used consistently for that paper/printer combination. (Caveat - You might find that you choose different settings for pictures where you have made a creative decision that don't have/ don't want lots of fine detail, such as portraits, although I've always dealt with this, perhaps wrongly, at the creative sharpening stage).
If anybody decides to do this / or knows where such practical experience with settings have already been documented / I'd for one be curious to know what the Aperture print sharpening settings generally seem to come out for both matt and gloss papers. Probably because my experience is with Photoshop, if and when print sharpening (and soft proofing) is critical, I switch to these. But there are lots of things I like about Aperture (plus lots of frustrations) and it strikes me that if you are starting out, and/or your budget doesn't stretch to photoshop (or you'd rather put your money into printers/ cameras/ lenses), providing your can do colour calibration of the workflow (borrow the equipment!) and providing you have the curiosity to experiment, and providing you don't use or experient with many different papers, you could do print sharpening and soft proofing adequately from within Aperture (which wasn't possible up until Lightroom 2, which was the last version I used). By adequate I mean such that the vast majority of people looking at the prints will notice a difference. I do like to have a working theoretical understanding of what is going on - but it is also important to not be afraid (or, worse, paralysed into inaction!) to just go out and gain practical experience and evaluate the results, which in the end will always be a value judgement about what pleases you and others the most - Discuss!
all the best, Malcolm