What is the best thing to buy/or do to clean connections on IC's and SC " ??

DeOxit, for me.
How important? Hmmm, not very.
B

I use DeoxIT Gold, in its bottled version, on anything I install in my system, and perhaps every year or two otherwise. I wipe off the excess with Q-Tips.  Seems to be an adequate approach as far as I can tell.

Best regards,
-- Al

I’ve been cleaning contact surfaces with Q-Tips and cotton pads moistened with denatured alcohol and then apply a thin coat of G100 Gold contact enhancer.

Engerene. not sure if its still made. Liquid naphtha in a can. Theres other brands as well. Many years ago, the best high end repair company in Seattle, Precision Audio, would sell from time to time used solid state and tube gear on a rack in their lobby. They would do full restoration and sold McIntosh amps from time to time. The old stuff. I was amazed how beautiful the old models looked, almost like new. I asked one of the technicians how the heck do you make the old pieces look so beautiful? He replied.."Engerene..its liquid Naphtha in a can..the chemical they use at dry cleaners". I said are you serious? He says Yep! It works better than anything we tried. Its great for cleaning metal surfaces and makes chrome and stainless steel look like new and does not damage or mar  lettering or numbers on the faceplates. Really stinky stuff. Use in a well ventilated area.

Thanks Guys , a LOT of good info , should take care of business !