Sandbagging is a superpower within reason under many circumstances. Most people would much prefer to get a quality promised deliverable on time or a bit early even if it's a bit longer than they would have preferred in an ideal world. (And, if they really do want to pull something in, you sigh and say you'll do your best but no promises.) If something really has a hard aggressive deadline, OK maybe.
But usually it's a case of it will probably take me this long to do a task. But there are some unknowables and someone I might have to lean on could have a sick kid for a couple days. Generally, everyone is happier if you underpromise and overdeliver including you.
An engineering manager I used to work with would drive me crazy because he had this idea of 90% schedules he got from somewhere. Which basically meant there was a 90% chance of meeting the schedule if nothing went wrong. (Which naturally mostly never happened.)
Sandbagging can also lead to nothing ever being worth doing.
Of course, it is shocking how many projects are started without anyone knowing anything other than the end date.
In my view, if the project is of such marginal value that a precise estimate is needed, don't even start the project - instead, find something more valuable to work on.
Maybe that's true of major projects, especially when there are maybe unknown unknowns. (And maybe not.) But there are a ton of deliverables that should be fairly predictable that come with people who have downstream dependencies on them. (Promotion, launch plans, etc.) When something is late it can cause a lot of scrambling and wasted time/money.
[And just to add, I'm mostly speaking to work I'm creating and delivering--for the most part individually. And to the degree that I'm depending on client reviews, etc. that's in the contract.]
The 90% schedule is not a bad idea, but you need to look at it via metrics - what percentage are you actually meeting that schedule? If you are not doing so 9 out of 10 times, you are at a 20% schedule or a 10% schedule.
But usually it's a case of it will probably take me this long to do a task. But there are some unknowables and someone I might have to lean on could have a sick kid for a couple days. Generally, everyone is happier if you underpromise and overdeliver including you.
An engineering manager I used to work with would drive me crazy because he had this idea of 90% schedules he got from somewhere. Which basically meant there was a 90% chance of meeting the schedule if nothing went wrong. (Which naturally mostly never happened.)