Interesting - and something to consider. I guess you are cutting steel roofing that has a zinc (or other galvanic material) coating (or layer under a prefinished surface) to retard rusting. Cutting through it with a circular saw or grinder - would indeed leave an edge/area that would be unprotected and be subject to corrosion. There might be issues too with cutting anodized prefinished aluminum.
For anything but small roofing jobs - we'd leave - or subcontract that work to the experts who did it regularly. For metal roofing. they had the specialized tools to crimp standing seams etc. When I talked about cutting roofing with our Evolution/Morse saws, I was thinking about work we did cutting corrugated decking and roofing that was used as a substrate for poured concrete. It was not subject to weather.
For flat sheets - most shears or nibblers will work as long as it can handle the material gauge. I do know, however, that not all shears or nibblers can deal with corrugated materials - especially if the corrugations are tightly-spaced. . For corrugated materials. it would still be best to try out a shears or nibbler on the material you need to cut