I don't use a hydrometer much, but I use it without fail when checking FG at the end of fermentation. As mentioned in a previous post, this isn't a completely predictable process, and the only way to be sure that fermentation has finished where you want is to check the specific gravity.....and it's not like it takes more than a few minutes to do it. Better than bottle bombs.
“Malt does more than Milton can / To justify God’s ways to man”
-A. E. Housman (1859–1936). A Shropshire Lad , 1896.