Is it possible to unselect Shipping method every time we visit checkout page please? I think it shows preselected option if I visit that page?

This is going to vary by theme. At a minimum, you could run a JS/jQuery command to switch the selected input for shipping option. There is also a configuration for this in your SC domain configuration, but I can’t remember if there’s an option to force a selection each time.

Yeah, while you can unselect a default shipping method in the website setup record, I don't think this will always unselect the option. It just sets the default to blank, which will probably work for new customers, but not for returning customers or customers who have set a shipping method already. So yeah, I think the best option may be just to write a simple script that unselects, if that's really what you want to happen.