In May, some rain can be expected in Bali and Lombok and much of Indonesia once every 4 days or so. May is a great time of year to travel to Indonesia.
Sumbawa, Flores and Komodo Island have climates similar to Bali.
The islands of Nusa Tenggara should see long clear days of sunshine and little rainfall.
May marks the beginning of the dry season in Java, Sumatra and Sulawesi. The odd afternoon shower may be expected during May but generally expect long clear days of sunshine.