I know that images need to be around 300dpi when printing, to ensure optimum quality... but do I need to actually specify that in Photoshop? (in the 'Image Size' section)
Because InDesign already states the 'Effective PPI' value in the Links panel... can I just go by that or is it still best practice to set the images to 300dpi anyway?
It has to be set to something in Photoshop, but it doesn’t matter what, particularly. All that matters with regard to final print quality is the number of pixels in the image (ie. its pixel dimensions) and the amount of space on the page that those pixels are being squeezed into/spread across.
The difference the ppi value assigned to the image will make is in how big it will appear on your InDesign page when you first import it. If it's set to 72ppi then it’ll arrive using that ‘pixels-to-physical distance’ ratio. In other words, an image that is 300px wide and set to 300ppi in Photoshop will arrive in InDesign 1" wide on the page, with an effective ppi value (in the Links palette) of 300. The same image, if set to 72ppi in Photoshop, will arrive on the page 4.17" wide with an effective ppi of 72.
So you can save yourself a step by setting the ppi in Photoshop and not having to scale the image down each time it gets imported into InDesign. But other than that? No, you can leave the photoshop setting at whatever you like. Just keep an eye on that Effective PPI value in InDesign.
To put it another way: whatever the PPI is set to in Photoshop will be the ’Effective PPI’ that the image arrives with when you first import it into InDesign.
Well, no you dont need to specify it. Its just that photoshop is unable to live with having not set this value. So it has to be set to something.
OTOH, if your using a different PPI in the file and effective PPI in indesign regularily. Then indeed there is no point. Good luck getting distance based measurements match across the software, though. But YMMV.
PS: 300 PPI isnt optimum quality, its the halftruth you tell children
