It is standard practice to use 16:9 menus if you have 16:9 video. Oh, course you don’t have to if you don’t want to, the disc will play fine with just 4:3 menus.
When you make a 16:9 menu you have options on how it will be displayed on 4:3 TVs, Letterboxed (so the menu will look exactly like video with black bars top and bottom and it scales the buttons overlays to match or Pan and Scan with just center crops the menu. So if you design the menu so all the buttons and important graphics are in the middle this gives you the cleanest look.
The only issues of the mixed aspect ratios are on 16:9 TVs or Computer screens, Menu of different aspect ratios then the video kind of glitch when switching but there is nothing technically wrong with them. If you use 16:9 you eliminate most of this problem.