I also prefer to check manually.
I also downloaded the High Sierra installer, not to install it now but to zip it up and stash it until I'm ready. What I did was when it finished downloading I pressed Command+Q to quit the installer app. It shouldn't bother you after that but in any case mine can't... it's in a zip archive...
I found that in link online, people often do this so they can create bootable flash thumb drives so as to install it on several machines instead of waiting for it to download on each machine.
Actually my iPad does this to me now and then... on starting it up it prompts me to install the new OS... I found that clicking later doesn't really do a lot, it's clear it expects you to choose a time whatever. So after I get through with that I go to system preferences under the general tab I can select to check storage on my drive... NOT iCloud... scroll down til I see the iOS 11 update and delete it. It won't bother me again for some time, usually after I've synced with my MBP. I haven't got my iPad on right now so I hope I remembered that place to find it correctly! I know it's the General tab and select storage, there are 2 options... on your iPad or iCloud and it's the internal drive not iCloud.
Anyway if you download an OS and don't want to install it right away, Control+clicking on the app to get the contextual menu, choose Compress "whatever app"... you will get a file: App Name.zip or if it's several items, Archive.zip which you can rename. I have a Install macOS High Sierra.app.zip in my Utilities folder. Then delete the app and empty trash. It can't install until you unzip it and put it back in the Applications folder and start it.