CAPT OF AN EMB120 FAILED TO FOLLOW THE TAXI RTE FROM THE RAMP FOR TKOF RESULTING IN ATC INTERVENTION WHEN AN MD80 ON A PUSHBACK WAS STOPPED AS THE RPTR APCHED.

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PARKED AT XX; RAMP CTL CLRED OUR ACFT TO 'TAXI E; TXWY C TO RWY 19L.' 4 GATES E AN ACR Y MD80 WAS PUSHING BACK WHEN THE RAMP PERSONNEL SIGNALLED TO HALT THE PUSHBACK; AS WE WERE NEARING ITS POS (WITHIN A FEW HUNDRED FT OF THEIR GATE). I THOUGHT IT ODD THEY WERE CLRED TO PUSH BACK WHEN OUR TAXIING ACFT HAD ALREADY BEEN PREVIOUSLY CLRED. JUST THEN; MY THOUGHTS WERE SHATTERED BY RAMP CTL AS HE AGAIN REITERATED OUR TAXI CLRNC TO RWY 19L VIA TXWY C AND WONDERED WHAT WE WERE DOING ON TXWY D DISRUPTING THE ACR Y PUSHBACK! ALTHOUGH THERE WAS NO DANGER OF A COLLISION; WE DID VIOLATE OUR TAXI CLRNC. HOW THIS HAPPENED WAS AN INTERESTING AND THOUGHT PROVOKING SELF- EXAMINATION THAT REVEALED TOO MANY ASSUMPTIONS WERE MADE. AS WE RECEIVED OUR TAXI CLRNC; I WAS ON THE RADIO WITH COMPANY. UPON MY RETURN TO RAMP FREQ I CAUGHT THE END OF OUR CLRNC TO RWY 19L AND PROCEEDED TO TAXI TO RWY 19L ON TXWY D; THE TXWY WE NEARLY ALWAYS USE. THE NEW FO ASSUMED I KNEW WHAT I WAS DOING -- I HAD BEEN WITH THE COMPANY 4 YRS AND IN AND OUT OF IAD DOZENS OF TIMES. AND WHEN THE ACR Y HALTED ITS PUSHBACK; I ASSUMED INCORRECTLY IT WAS FOR US; NOT BECAUSE OF US. THIS COULD HAVE BEEN AVOIDED INITIALLY HAD I MERELY ASKED THE FO UPON MY RETURN TO RAMP FREQ TO VERIFY THE TAXI CLRNC. MAKING ASSUMPTIONS IN THIS CASE HURT MY PRIDE AND RESULTED IN A LACK OF PROFESSIONALISM BEING DISPLAYED AT WORST. IN OTHER SITS IT COULD BE MUCH MORE SERIOUS. ASSUME NOTHING!