PVT PLT OF SMA ACFT OVERSHOT ALT DURING DSCNT DUE TO CONTEMPLATING A USUAL CLRNC AND BEING DISTR BY AN UNEXPECTED CLRNC.

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ON AN IFR FLT PLAN TO PAO (IT HAS NO APCH BUT IS USUALLY COVERED WITH THE BAY STRATUS IN THE SPRING AND SUMMER). I USUALLY SHOOT THE APCH AT SAN JOSE TO GET UNDER THE STRATUS (TOPS USUALLY 2500-3000 FT AND BASES AT 1000-1100 FT) AND THE CONTINUE RNAV OR VECTORS TO PAO. THIS HAS WORKED FOR YRS. THE WIND IS USUALLY FROM THE NW AND FAVORS THE ILS FOR RWY 30. ON THIS DATE HOWEVER THE WIND WAS FROM THE SE AND RWY 12 ILS WAS IN USE. BAY APCH (135.4) HANDED ME OFF TO S BAY SECTOR FREQ 120.1 ABOUT 12 MI FROM SAN JOSE AND I WAS LATE IN GETTING CURRENT ATIS AND ASSUMED IT WOULD BE ILS RWY 30. BEFORE CONTACTING 120.1 I OBTAINED THE ATIS AND IN MY RUSH TO SET UP FOR THE ILS RWY 12 I DROPPED FROM ASSIGNED 6000 FT TO 5840 FT. AS I CHKED IN WITH 120.1 I THEN NOTED THE 5840 AND RPTED LEVEL 5800 FT. THE CTLR ASKED MY ASSIGNED ALT AND I STATED 6000 FT RAPIDLY RETURNING TO 6000 FT. THERE WAS NO CONFLICTING TFC AND VISIBILITY WAS UNLIMITED ABOVE THE RAPIDLY BREAKING LOWER OVERCAST. I WAS ABLE TO FLY TO THE W SIDE OF THE S BAY AND PROCEED VFR TO PAO. THE CTLR CONTINUED TO PROVIDE VFR ADVISORIES. I FELT I SHOULD HAVE OBTAINED THE ATIS MUCH EARLIER INSTEAD OF MAKING THE INCORRECT ASSUMPTION ON THE 'USUAL' APCH. I ALSO SHOULD HAVE USED THE ALT HOLD ON THE AUTOPLT (IT WAS ON HDG 'BUG' BUT NOT ALT HOLD). ONE SUBTLE INFLUENCE -- THE IRS AND I ARE 'DISCUSSING' MY FLYING -- THEY AGREE WITH THE BUSINESS USE BUT ARE DISALLOWING 'IFR PRACTICE.' I USUALLY FLY PRACTICE APCHS 2 HRS PER MONTH BUT HAVE BACKED OFF TO THE MIN FOR CURRENCY TILL THE APPEALS HAVE BEEN COMPLETED.