My simple question is that, Do i need to compile less with grunt in magento? My theme is working fine. However i am not sure that i need to compile less with grunt or not. Is there any impact on performance ? Please suggest what is best method ?
There will be no performance gaps. Official Magento documentation offers this way. More details here: https://devdocs.magento.com/guides/v2.3/frontend-dev-guide/css-topics/css_debug.html
The short answer is no
The longer answer is that you can do although I've experienced some issues using grunt. Grunt is faster and at times gives more meaningful error messages.
The main issue is that it doesnt factor theme inheritance the same way static content deploy does. Which can lead to errors.
So personally I don't recommend using grunt. Certainly nothing beyond development.