When you select an image in ID and check the info you'll see two resolution values, the Actual PPI and the Effective PPI. Effective PPI is the resolution at the size you are using the image on your page, and it's the one that matters.
300 ppi is a rule of thumb, and you can get away with less for most images, and a lot less for some. Resampling in Photoshop to bring the effective resolution up is generally a waste of time -- it will not improve the image quality. If the effective resolution is below 200 you probably need to reduce the physical size of the image, or find a higher resolution version. If it is above 300 ppi it can be downsampled during the export to PDF, so there's no need to mess with it in photoshop unless you are downsampling, then tweaking the image in some way before replacing it in the layout.
To add to what Peter said, there are two things that can happen when you place an image. One is if you just click on the page, and the image is placed at 100%, and the other is if you place it into an existing frame that has scaling applied, or if you click and drag. In those cases, the scale will depend on several factors, which isn't really important if you understand that it isn't 100%. The only reason 100% is important is if you plan to place images that need to be a certain resolution that you have already set in Photoshop, and you know you don't want to scale them in InDesign. That's a fairly limiting way to work, but if some people want to work that way, it's no skin off my nose.
Since you can downsample resolutions above a particular number in the export to PDF stage, you could use this workflow to place images at 100% and only scale them down (which increases their effective resolution) as a way to ensure that they are 300ppi at their lowest, and if that's what you would like to do, there is something that can speed you up a little.
Since your images are getting smaller when you up-res them in Photoshop, I can assume you are doing so without resampling (and that's good). You aren't losing any image quality going from 72 to 300, but when placed at 100%, they will be smaller in ID. If they don't need to be scaled up in ID, you can use them, and if not, you may consider getting new images. You can do all of the upscaling with a Photoshop action or droplet for all of your images at once if you feel that would be of help to you. As Peter said, it's really not going to be better, just a different way to work, and if you like it that way, you aren't really losing anything. And, as Peter also said, 300ppi isn't really a magic number.