Are the tool rests on budget bench grinders any good?

Generally no. Across r/turning and r/Tools, the most consistent criticism of inexpensive grinders is flimsy, out-of-square tool rests that are not level with the wheel. Many owners replace the factory rests with an aftermarket sharpening jig or platform regardless of brand, so it is worth budgeting for that upgrade if precise sharpening is your goal.