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If you do not have to report a change in altitude and start decending, will ATC see that I am decending and automatically issue a traffic advisory if traffic is below me is on a similiar course?
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There won't be any (IFR) traffic below you. If there were, you wouldn't get a Cruise Clearance. VFR traffic you are on the hook to "see-and-avoid" (like always).
Also, you may not even be in radar coverage. Cruise clearances are a lot more likely way out in the boonies where there isn't much traffic and not much expected, and often radar coverage is slim or non-existent.
Hmmm.....I say that like I know what I'm talking about, but I've never received a cruise clearance. I wonder how often they are used these days, and if they are, where they are used?