Its is something which is not mandate.
Start up folder consist of the item which will start or launch when your system will start hence if you want the Adobe Reader to launch on every start you can have it there.
However if you are concerned regarding the Adobe Reader or Flash Player functionality in the Web browser then no need to worry , there are plugins which intiaite this, hence no need to put them in the startup folder.
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Anything that should be in the Startup list will be put there by the installer.