Should I reduce jpeg dpi down to 300 before placing?

I've been given over a dozen jpeg headshots in random sizes and DPI counts I need to design with. They will all need to be reduced in size in my layout. Some will end up about 300 DPI, others will be much higher when reduced.

Should I reduce them individually to size and convert them all to 300 DPI in a separate paint program before bringing them into my Illustrator layout...or can I just bring these random photos into my layout file, reduce them there, and let Illustrator convert everything to 300 DPI when I export a PDF for the printer?

Thanks!