It depends on the type of programme you are trying to uninstall.
If for instance it is mobile phone software, that comes with drivers for hardware and if not properly uninstalled can lead to trouble and BSODs in future. Best to perform good removal practices.
If it's a game, just deleting should not cause any real harm to your OS. A good removal will leave your machine in a more optimal state.
I have 'fixed' many friends computers that have 'gone slow' over the years and it's generally the dross left over from old programmes (whether uninstalled or just deleted) and unused programmes residing on the machine that make an impact.
Limiting all the extras run at start-up helps to but that's another topic.