When I first started with digital photography around three years ago it was with a pretty decent compact, a Canon A95. For the first year and a bit I sharpened every single of my images by a factor of 150% & pixels at 3-4 ignore threshold and 3-4 radius, sometimes more in addition to boosting the contrast and brightness by 40/30 or more..
The resulting images looked great on my monitor and didn't turn out too shabby on 8x10 prints either, while for web they were ideal... However.... when I started looking at selling my images via microstock I got a whole lot of rejections (over 90%) due to over sharpening and over filtering. Only the best images made it through and even those I would never submit now days.
Thus the learning process commenced.....
What I learnt is that by using the unsharp mask to heavily you introduce artifacts or not so much as introduce but accentuate. Therefore the lower the quality of the digital sensor on your camera and the quality of the lens used, the higher the number of artifacts in any given image. The only way to ever examine the quality of an image is at a 100% 1/1 ratio and using a proper image editing software such as photoshop. It is a fine balancing act and even nowadays I still get an occasional rejection due to over-sharpening or artifacts. This is also because a lot of my images are HDR where you sandwich several different exposures into one, a process which if you like triples or doubles the number of artifacts that you end up with. However I still submit my HDR's to microstock and have a pretty high acceptance ratio at most agencies.
Its also worth noting that all digital cameras nowadays already do some degree of software driven sharpening at the capture stage inside your camera. Therefore it is important to understand just how much your camera does before you put the image on the editing table. The key thing is to keep in mind is your final intent for the image edited.
If you want to use your images for the web and are not fussed about going larger then an 8x10 print or selling your shots then you can quite easily go up to 150% & pixels at 3-4 ignore threshold and 3-4 radius, providing this holds up to a brief visual inspection at 100% ratio. If however you wish to leave the possibility of doing larger prints or selling your work then a number of factors need to be carefully considered:
1 - The quality of the lenses that you use (more on this below).
2 - The quality of the camera's sensor (more megapixels doesn't equate to better sensor nor better images, find out about your camera from http://www.dpreview.com/ ).
3 - Stability or the dreaded camera shake. Don't use a shutter speed lower then the focal length that you are shooting at and if you do make sure you have invested in a very robust tripod or have faith in your IS or image stabilization on either lens or camera.
It is for this reason that I only ever use the best lenses possible in terms of quality, reading carefully all lens reviews and buyer feedback( bookmark this site http://www.fredmiranda.com/reviews/ ) and making an informed decision before each and every single purchase, be it $100 or a $2000 lens. I have bought lenses before for around the 1k mark only to find out the quality of the final result was sub-par and below my expectations, rendering those useless to me. Luckily I managed to sell these on E-bay with minimal if any financial loss, hoping that the buyers would be satisfied with the quality they got for their own needs. I also make sure that I use a camera with a good quality sensor, or as good as I can justify and afford, but never at the price that would force me to compromise on any of my lenses budget wise.
A standard prime (non zoom) lens will almost always produce a superior results to a zoom lens. I know this as I have conducted many tests in controlled conditions over a variety of lenses. Not to say that all zooms are bad, there are some that really stand out like the Canon's 70-200mm L (any of the four available F2.8IS, F2.8, F4 and F4IS respectively) series. The quality of the images produced by these lenses is quite simply outstanding for a zoom. Canon have really nailed it with this one in terms of optical alignment and calibration.
Off the lens mantra and back on topic, indicatively speaking and from my own experience I now never sharpen by a factor higher then 55% & pixels at 0 ignore threshold and 1.5 radius. This is unless of course I'm making an avatar or a web only image in which case I might go as high as four times that. I also do not use the unsharp mask unless I really feel it is absolutely necessary, for instance when I might be trying to compensate for lower focus, lower depth of field or even the occasional camera shake. I only ever do this if I really need the image. Furthermore I hardly ever sharpen images with any visible noise, ie low light or HDR's. Thus my final advice is to know your equipment, shoot wisely and always inspect the final image at the editing stage at 100% ratio, corner to corner before you save and index. Have fun and I hope this helps