Usually using GND for analog signals is perfectly fine. In most cases it makes little or no difference. But in some cases, like PWM to LEDs or motors, separate grounds for analog and digital can reduce noise. Even in those cases, using GND is ok, if you're willing to accept some extra noise.
The opposite is not true. Never use AGND for digital signals. There's an inductor between AGND and GND, which isn't good when you have fast changing low-to-high and high-to-low voltage of digital signals.