Generally speaking, would you say that most people can be trusted or that you cannot be too careful in life?

Trust in this context can be defined as the firm belief in the reliability, truth, ability, or strength of someone. The consensus is that trust has to be earned. What this means is that the person to be trusted (the "trustor") has to have acted, over a period, in a manner that has convinced the trustee that he or she deserves trust. The situation does become complicated, however, when one is involved in social situations in which the trustor and trustee have not had consistent contact or have never been in contact at all, as in a first-time meeting. In such a context, it can be said that having "a firm belief" in a stranger would be foolhardy and naive because the trustor has not had occasion to earn trust. The trustee will, therefore, place him or herself at risk by trusting such an individual or individuals.
The notion that "one cannot be too careful in life" then becomes a reasonable option. For one, it reduces the risk of the would-be trustee's abuse or exploitation by others and ensures his or her safety. It is, therefore, essential that an individual is always aware of the risks of blind trust. One should consistently consider one's own best interests and carefully assess the outcomes of one's actions, especially if it involves placing trust in another, whether the individual is known or not. 
History has emphatically proven the irony of placing trust in figureheads and those we believe have more authority, knowledge, compassion, care, understanding, and so on than ourselves. In many instances, the trustees' faith has been exploited by an unscrupulous individual or individuals who have sought only to achieve their agendas. It is unfortunate that a cynical or suspicious approach should become a rule of thumb. However, as the cliche goes, "it is always better to be safe than sorry."