Look up your ATHR logic. In Managed speed the ATHR will give a thrust setting for the flap lever position. With Slats/Flaps locked will you get the normal config when you move the lever? Better to be in Selected speed as you now control what speed the ATHR will command - less likelihood of the ATHR giving an inappropriate speed for your actual slats/flaps configuration. The Go Around can be interesting in such situations.