Well I keep doing so too sometimes on some of the posts, but not very actively.
However, I never tried changing the posts title tag text.
Last days (2 weeks ago) I decided to try that. I changed the title of one of my blogs post to include the keywords exactly as the most searched keyword for that post and the one which was giving me highest traffic.
Guess what . . . Google didn’t like it!
The traffic on that post, after optimizing its titile, went down almost 50% and it was really a shock to see!
So definitely I removed that title and gave it the original one, and luckily, the traffic went back to normal, but after almost 7-10 days (whereas the drop was almost instant)
So be careful when you make changes to your posts (Specially titles and prominent keywords in it)
You might like to make a similar post with the title you want, which will not affect the current post and its ranking in Google Search.