Remember that people's visual acuity varies quite a lot, as does their expectation of image quality; and the actual dpi that the printing machine will print at also varies. The various LED and laser printing machines that expose photographic paper typically have a native setting, or in some cases settings, of around 200 dpi; 300 dpi; 400 dpi; 600 dpi or even 1200 dpi now. (In this case a single 'dot' consists of three coincident exposures, one each in red, green and blue.) If you wonder why 300 ppi is used as a common standard when 100 ppi is enough, ask yourself why bother with printers capable of 400 dpi or 600 dpi.
A file sent at 300 ppi may be printed at only 200 dpi on a machine with an exposure unti that can do both 200 dpi and 400 dpi because the machine runs faster at 200 dpi, and most people will be happy with the 200 dpi print. It should not be difficult to find out what dpi your file will actually be printed at (and you should also be able to get the printer profile for the machine your image will be printed on, of course).
Inkjet printers often work best at ppi values that are simple divisions of their dpi setting - eg if you have an Epson that prints at 2880 dpi it will probably be optimal at 288, 360 and 720 ppi, and some people will be able to see the difference between the 360 ppi print and the 720 ppi print, if the original image has real detail at the 720 ppi level. Whether you need that extra resolution is an entirely different matter.
Inkjet printer resolutions and LED/laser exposure unit resolutions can't be compared directly, because the image is made up in two very different ways.
Some people find that they do not require an absolute resolution (pixel dimensions) greater than the equivalent of 10 x 8 at 300 ppi - ie pixel dimensions of 3000 x 2400. As the file is printed larger, the ppi drops, but so does the likely viewing distance. On the other hand, some images invite the viewer in when they are printed large, and in that case you may wish to keep the true resolution high so that as the viewer is drawn into the image it does not fall apart, but reveals more.
Rather than work to a fixed quality used by someone else, it might be worth doing your own testing to find the print quality you prefer for your images at the size you want with the printer or printing service you use. You can also test the difference between doing your own interpolation and letting the printing service / printer driver do the interpolation - it may be acceptable and easiest to leave the interpolation to the printer.
In some cases you may be asked for images with specific resolution standards, particularly for magazines, brochures etc. Often that requirement will be 300 ppi even though the printing method may not be capable of resolving that level of detail - the oversampling makes sure that the final image is as good as it can be.
