I leave documents as PSD files. Unless you want to use a compressed TIFF format which used non-lossy compression. Years ago, I always used TIFF. But anymore I leave files in native format. If you are sending files to someone else, then send TIFF because they may not have Photoshop. I try not to send files with layers to a client because that lets them edit in ways I’d prefer they didn't. If edit are needed, this encourages them to come to me.
Saving as TIFF files is fine. But not necessary to bring the files into InDesign, Illustrator or even Quark. You can even drag & drop a PSD file into Word.
On somewhat of a tangent:
I’d like to remind people that images in InDesign are not embedded. So if you lose or misplace a file that is in a document, it will print as a low res preview or it may not print at all. A good thing to do is, once you have finished your work, make it a habit to package your project (File > Package). This collects the final file, instructions, image files, and fonts into one folder. This is what you would give a printer (this or a PDF). The instructions simply describe the file - page size, images, placement of images, etc. You don’t need it but it can be handy for the printer.
I would also recommend Type 1 (PostScript) fonts or OpenType fonts, OpenType fonts being ideal. OpenType fonts use the exact same font file on PC or Mac. So there should be no shift in type layout due to using a PC version of Helvetica and then shifting to a Mac version of Helvetica. Regardless, I recommend that you stay away from using Windows TrueType fonts on a Mac. Yes they run on a Mac just fine. But I had a student do this on a project, and when she went to print it, all the type defaulted to Courier and it was a mess. So at the very least, test a Windows TrueType font if you want to use it, and be aware that while it may print from your printer just fine, a printing bureau might still have problems. And this is why I also send a proof with a job - so the printer knows what it SHOULD look like. When in doubt, ask your printer.
InDesign has a LINKS panel that provides great info. You can find out the color mode of an image (should be CMYK for print, RGB for web or on-screen viewing), resolution, size, etc. You want to pay attention to the EFFECTIVE resolution. If you have an image that is 20 x 20 inches and 72 dpi… and you place it in your document at 5 x 5 inches, the effective resolution will be 288 dpi. Not quite 300 dpi (best for print projects), but close enough.
When exporting to PDF, the advantage is everything is embedded. But be sure to save your InDesign files in the event you need to go back and edit something. AND in PDF settings, make sure you save for the project you have in mind. For print, use either PRESS QUALITY or HIGH QUALITY print. Or better yet, create your own preset. I was taught by people who work for various publishers to basically make a preset where nothing is compressed or downsampled. For posting a PDF on a web site, you’d want a smaller file size, so I would then use compression and downsampling.
And if anyone is in the position of receiving low res images from clients, and having to increase resolution (which tends to produce fuzzy images), there are several options. One is what I call the 10% rule. Found here: http://karenackoff.net/class/increase/increase.html
Or... ON1 RESIZE 2018 - which I think evolved from Genuine Fractals. This takes a low res image and makes it high res and keep amazingly good quality. While Genuine Fractals used to be several hundred dollars, RESIZE is currently selling for $60. A good deal. And for both PC and Mac.
I’ve been teaching Photoshop, InDesign and Illustrator lately… so these issues have kind of been in my face. Apologies for the rant. And please always remember that your mileage may vary. This is what works for me.
Side question: Karen, when you flatten your PS files, do you just keep them as native .psd files? I’ve always flattened, then saved as .tif’s.