I see the risk. However, after 4 years at the company, I felt I should have been trusted. Here's the rest of this particular story:
A year later, I said screw it and worked on it anyway, for 2 weeks (manager was out of town). It was an internal tool. It became the most popular internal tool in the office (95% of people have it open in a browser tab all day). At one point, the founder checked it out and said "I love [name of tool]!". Vindication. I left the company after that, and they use the tool to this day.
I've had a similar experience several times as well.
There's a real disconnect when you are chewed out for having wasted company time on improving a process and it is still used years after you have quit the company. (I actually managed to replace about 20 hours of my work week with several scripts that took two or three minutes to run. The best part is that I also did the same for a project manager - and he would regularly get bored web surfing and come and ask if I ever needed any help.)
That's what project management is there for, at least I was told that. You can't replace simple routines by a script, you know you made this guy out of work? ;-)
Disclaimer: Not at all serious, except the first sentence.
A year later, I said screw it and worked on it anyway, for 2 weeks (manager was out of town). It was an internal tool. It became the most popular internal tool in the office (95% of people have it open in a browser tab all day). At one point, the founder checked it out and said "I love [name of tool]!". Vindication. I left the company after that, and they use the tool to this day.