Link posted by someone on another forum, but IMO an interesting analysis by Spencer Cox.
The article is specific to Nikon cameras (per the author). However the principles are applicable to all of us to one degree or another.
From my own perspective as an Olympus user, the comments about cropping and ISO are somewhat different.
In Olympus land, almost nothing done to the JPG settings has any effect on the resulting RAW file, but none of my Olympus cameras are anything like ISO-less! Closest is my E-M1, which is relatively stable between base ISO and around ISO 1600, then again from 3200-6400, with 12,800 and 25,600 being somewhat "individualistic", shall we say !
Olympus makes some use of on-sensor analog/digital converters (I think ... ), and then uses some digital jiggery-pokery after the image is converted into digital form (chicanery?) in some of their cameras (e.g. my E-30 does this ... ). It is this latter process that causes some Olympus cameras to become very noisy at their higher ISO settings.
IMHO, this is a decent article that warns us all that we should at least attempt to understand what our cameras are doing behind our backs! And what the ramifications are for our use of our camera/s of choice, both in-camera and in PP.