NEW PLT EXPERIENCED CONFUSION REGARDING VFR FILED ALT AND ATC ASSIGNED ALT TRANSITIONING PHX CLASS B AIRSPACE.

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AS A BRAND NEW PLT (JUST RECENTLY PASSING MY CHK RIDE) FLYING ON A XCOUNTRY; EXCEPT FOR JUST TRANSITIONING DIRECTLY ABOVE THE PHX ARPT BTWN N AND S; I HAD NEVER REQUESTED TO FLY THROUGH THE PHOENIX CLASS B AIRSPACE BEFORE. I COULD HAVE FLOWN UNDERNEATH THE AIRSPACE BUT IT SEEMED TO ME THAT IT WOULD BE SAFER TO GET RADAR FOLLOWING; FLYING THROUGH THE CLASS B AIRSPACE. SO AFTER CHKING THE WX I FILED A FLT PLAN; FOR AN ALT MORE THAN 500 FT BELOW THE BOTTOMS OF THE CLOUDS APPROPRIATE FOR THE DIRECTION OF FLT. SHORTLY AFTER TKOF; I OPENED MY FLT PLAN CHOOSING AN ALT SUGGESTED BY FLT CTR AND THEN CONTACTED ATC TO REQUEST CLRNC TO FLY THROUGH THEIR CLASS B AIRSPACE AND SHORTLY THEREAFTER RECEIVED CLRNC; ENTERED THE AIRSPACE; ASCENDING TO MY REQUESTED AND ASSIGNED ALT. AFTER FLYING FOR 15 MINS OR SO; AS I APCHED THE SOUTHERN END OF THE CLASS B AIRSPACE; I REALIZED SOME OF THE CLOUDS AHEAD WERE AT OR BELOW MY ALT; AND IT BECAME INCREASINGLY APPARENT THAT I WOULD HAVE TO RAPIDLY DSND. SO I TOLD ATC THAT I WANTED TO CHANGE FREQS TO MODIFY MY FLT PLAN. THEY APPROVED THE FREQ CHANGE AND I CALLED; WHEREUPON THE FLT CTR ACCEPTED THE CHANGE. I SWITCHED BACK TO THE ATC FREQ AND BEGAN DSNDING; NOT REALIZING THAT I HADN'T GOTTEN ATC TO OFFICIALLY APPROVE MY ALT CHANGE FIRST. BY THIS TIME I WAS PROBABLY OUTSIDE THE EDGE OF THE CLASS B AIRSPACE. 5 MINS OR SO FLYING WENT BY WHEN THE ATC CTLR CALLED ASKING IF I WAS DSNDING. I SAID YES. I WAS DEFINITELY OUTSIDE HIS AIRSPACE BY THEN AND HE SEEMED TO REALIZE IT SUDDENLY; SO HE TERMINATED RADAR FOLLOWING WITHOUT ANY FUSS. NO DANGER TO OTHER ACFT; NO INCIDENT OCCURRED; AND THOUGH THE FLT CTLR RELEASED ME IMMEDIATELY FROM RADAR; IN THE FUTURE; WHEN MAKING CHANGES; I WILL ALWAYS THINK -- ATC FIRST; FLT PLAN SECOND. ALSO; I WILL ALLOW A WIDER MARGIN FOR WX CEILING DIFFERENCES FROM PREDICTIONS.