Why your pull request might not be merged
00f.netI agree - if you are going to invest non-negligible time and effort into submitting a pull request on somebody else's project, ask whether your change is wanted.
I prefer the "don't waste more than 2 days writing the pull request" approach, only a mythical weekend, not hard feelings if it's not merged.
For me, in short PR it's easier to explain "I want to modify foo so it also does bar" with a pull request than in plain English.
If you are going to take a month to write a PR with a few hundreds of LOC changes, then I agree that it's better to ask the maintainer first.