Safety of volcano tours
An appropriately prepared volcano tour doesn't normally pose a large safety issue, although visitors to volcanic areas must be aware of different types of safety hazards that they can be subjected to. Before visiting a potentially dangerous volcano one should do some homework first and learn about how the eruptions of the volcano were in the past, what's the current activity of the volcano, if it is monitored and if there are any forecasts of increased activity. Finding out the current status is the key, not only for the safety, simply because the entire trip to the volcano may need to be cancelled if, for example, local authorities establish that it's too dangerous and restrict access to the volcano area.
Each volcano is different and each poses different dangers. Even hiking an inactive volcano should not be taken slightly as it may pose hazards typical to ordinary mountain climbing, like altitude sickness, quickly changing weather, rough terrain etc. depending on the type of volcano and its current activity, some safety equipment may also be necessary, like helmets or gas masks (sturdy, comfortable shoes are an obvious must-have on practically any volcano).
As each volcano is different, some are better to be viewed and enjoyed from far away, some are safe enough to climb to rim of the crater. Also, depending on the volcano and one's experience, hiring a local guide for a volcano tour should be considered.
For the ones who don't like hiking: when visiting an active or potentially active volcano, probably the safest way is to view it is from the air; in many places (like Hawaii, Yellowstone, Mt. St. Helens) there are tour agencies organizing airplane or helicopter tours that give the tourist a unique chance to get the bird's eye view of a volcano.