I have never heard of any unwritten rule or tendency to take a pitch after back to back HR's, which means there is no such unwritten rule or tendency....
What would be the point?....Fella gives up back to back homers and throws me one down the middle, you bet I'm going to try to get me a piece of the pie.....
Now after back to back walks, THEN you had better be taking the first pitch and if you don't, you had better knock the cover off of it.....
Now some pitchers take umbrage at a fellow swinging at a 3-0 pitch (especially if the team at bat is winning), but those guys are all girlie men......