The answer is yes and no. Tip plates fool the wing into thinking it has a longer span thereby reducing induced drag a little. They also add wetted area increasing form drag a little. Induced drag is mostly a factor at high angles of attack, whereas form drag is more of a factor at high speeds (low angle of attack). As for reducing stall speed, to the degree that the wing sees itself as having a longer span it may reduce it a little, but the tips add weight, which increases stall speed, so the net effect just depends. There are more simple ways to reduce stall speed, the most effective of which is reducing weight.

For the typical R/C model, drag isn’t really a major factor because they are so overpowered to begin with. In fact, for aerobatic aircraft, a little drag isn’t a bad thing as it helps control speed on the down line. 3D aircraft spend most of their time at low speeds where drag is not much of a concern either. Stall speed is only a consideration when landing, and if the airplane is reasonably light, landings are not a problem.

If you just want experiment by all means try tip plates, but if for aesthetics or other reasons they don’t appeal to you, be assured that you are not missing much by not using them.