Prepare marinade: place soy sauce, sherry, optional sake, unseasoned rice wine vinegar, brown sugar, minced garlic, minced ginger, sliced scallion, and crushed red pepper flakes in a large saucepan and heat through, whisking until sugar dissolves.
Let cool; transfer to medium mixing bowl, add thawed filet mignon tips, and marinate in refrigerator for 3 hours, turning occasionally.
If using bamboo skewers, soak in water for 30 minutes while tips are marinating.
Remove tips, reserving marinade, and thread onto skewers, using two parallel skewers instead of one to prevent tips from spinning and not cooking evenly.
Pour marinade back into saucepan and bring to a boil; cook over medium-high heat for 5 minutes, until reduced and syrupy.
Grill kabobs over direct medium-high heat for 8 to 10 minutes, turning occasionally while brushing with marinade glaze.
Plating: Remove tips from skewers and transfer to plates with a bed of rice.