There is no upscaling if display and render resolutions are equal. Be aware your display resolution could be some pixels smaller if overscan (config.txt) is enabled. So if render resolution is 1920×1080 and your display resolution is 1910×1070 (overscan) there could be scaling artifacts.
If performance is ok i would recommend render = display resolution. If performance is to low (shaders, emulation speed) a mix of hardware upscaling and lower render resolutions should be used. Hardware upscaling is for free and needs no processing power.
I have not seen any input lag with hardware upscaling. Thought threaded video causes a small input lag.