It is not dangerous to climb a volcano if it's not erupting. It's even more fun to climb such volcano, because you could witness the activity of the volcano (e.g burning lava at night, or see the mountain coughing vulcanic stuff every few minutes, or even experience small volcano eruption). The mountain ranger will prevent you to climb when it's dangerous.
I and my friends experienced the volcano eruption when I climbed Mount Rinjani two years ago.
The small volcano near Rinjani was erupted, not the mount Rinjani itself, of course.
It started with loud explosion almost every hour and can feel the ground shaking.
And finally, at night, about 2AM, the natural giant fireworks was revealed, with the volcano sending lava to the air. Fantastic!
Smaller scale of such thing can be witnessed daily on Mount Semeru too. It cough regularly and sends gas and volcanic ash every 15 minutes. Nice background for your summit victory photo hahahaha