How low before you need a C notch

[66 lwb cs]

I did a search but no luck so I was wondering how low can you go before you need to C notch the rear I am looking at a 3/5 for now maybe a 4/6 down the line but no more than that so I was wondering what kind of clearance there is in the back with a 5" drop spring will I need to notch and would it be better to go with the 5" spring or a 3" and a block and if so why. Thanks in advance for any info.