You should always aim to keep your ISO as low as possible. The higher it is, the more noise will appear in your shots which will affect the sharpness.
Whilst some people might recommend using the auto ISO setting on your camera, I would avoid it. Sure, the camera will try to keep the ISO as low as possible, but often that will end up being higher than you actually would want.
Instead of using auto ISO, learn how to manually move your ISO up quickly. Most DSLRs will allow you to change your ISO using a button and a dial.
Try to get into the habit of adjusting your camera settings and ISO throughout the day. So for example, as you move from a shaded area to sunshine, tweak your camera settings.
Typical Settings for Different Scenarios
The important thing to remember is that these camera settings are just a guide.
So here are my favourite camera settings for 10 travel photography scenarios.