Leaving a USB flash drive plugged into the USB port all the time is usually okay as long as lightning doesn't strike your computer or a power surge doesn't come in an destroy you computer and USB flash drive.
I would recommend just unplugging the Flash Drive and store it somewhere safe such as in a desk drawer etc until you need it and label it with what data you have on the flash drive so it's easy to find the information you need.
I have flash drives in my computer desk drawer labeled with certain photos, some videos and others for databases, website files etc.
Leaving the USB flash drive plugged in might cause you to accidentally delete files or someone else who uses your computer may decide to accidentally delete the files as well.
I have fireproof lock box secured down in my desk drawer that I lock the USB flash drives in so that they are protected against theft as well as fire.
Also just to note USB flash drives do eventually go bad after about 10 years as they lose the electrical charge over that time that helps to store the files and photos on them.
So if you have important data that you need to keep for a long time or forever you need to have the data stored on not only one USB flash drive but also a few others as well as an external hard drive, computer hard drive and in the cloud through Google Cloud Storage or other storage.
I also recommend having the files stored in Google Cloud Storage as well because in the event you have a house fire or other loss of your house and you cannot get the hard drives out of the house or your computer gets burned up or destroyed then you would still have the information.
I keep a copy of my files on USB flash drives in my cars trunk as well so that I have backups of my files that I need.