if you are really lazy you can make a .bat file that you double click to switch out your custom folder in order to add/remove prec
so you make one folder and name it custom_with_prec or something and then another named custom_without_prec
then in order to create the .bat file you just open a notepad and put this in and save as a .bat or windows batch file
rmdir "C:\Program Files (x86)\Steam\SteamApps\common\Team Fortress 2\tf\custom"
mklink /j "C:\Program Files (x86)\Steam\SteamApps\common\Team Fortress 2\tf\custom" "C:\Program Files (x86)\Steam\SteamApps\common\Team Fortress 2\tf\custom_with_prec"
and name it prec.bat
and then make another one with this instead and name it whatever you want (prec_off or something should be fine)
rmdir "C:\Program Files (x86)\Steam\SteamApps\common\Team Fortress 2\tf\custom"
mklink /j "C:\Program Files (x86)\Steam\SteamApps\common\Team Fortress 2\tf\custom" "C:\Program Files (x86)\Steam\SteamApps\common\Team Fortress 2\tf\custom_without_prec"
dr shadowpuppet told me he uses a similar symbolic link for replacing his custom folder in order to do tftv casts but figured out the logic to have just one .bat file that toggles it
if you want to get really fancy you can also make shortcuts to launch the game in different dxlevels (for example, if you normally play on dx8 with prec but need to use dx9 and without prec in order to play matchmaking)
all you have to do is make a new shortcut (right click -> new -> shortcut) and direct it to steam.exe
then open its properties (right click -> properties) and change the target from
"C:\Program Files (x86)\Steam\Steam.exe"
to
"C:\Program Files (x86)\Steam\Steam.exe" -applaunch 440 -console -novid -h 1080 -w 1920 -dxlevel 81 -full
adjust accordingly if you have steam installed on another drive or to change it to whichever launch options you want, the important part is that the directory for steam is in quotes and afterwards you have -applaunch 440 to launch tf2
iirc having -dxlevel in your launch options after the first time you change it makes it take forever to alt tab (at least for fullscreen) so to use this you would just put the .bat files and separate steam shortcuts (for the launch options you want to change) into one folder and then in order to play mm you just double click the bat file, then double click the steam shortcut and it will launch tf2, then close the game and relaunch it normally from within steam (you may not need to do this if you use windowed)