I like to check for leaks "before " I put the system in service , so if I do have a leak I dont have to recover and re evacuate ,But I do this for money so time is a big factor to me.If yor doing this in your spare time on your own equipment it probably doesnt matter.Any effort to clean ,evac,or check at all will be more than you get at a lot of shops around here.(I know of several contractors around here that use old compressors for vacume pumps,or dont evac. at all)If you get the air out and the refrigerant in it will cool if all the parts work.The other stuuf (nitrogen purge,evac,leak check etc... ) just ensure it will cool for longer and more efficiantly