While measuring tyre pressure, it is a good idea to perform a quick inspection of the tyre condition to ensure that the tread and sidewalls are in good working order. First, inspect both sidewalls to ensure that there are no splits or cracks caused by impact or perishing. Looking at the wall bulges makes it easier to run your hand around each sidewall. The tread should be examined; the minimum safe tread depth is 1 mm, but it is prudent to consider double this as the minimum for proper operation. If the tread is unevenly worn and the tyres are properly inflated, the suspension or steering may be misaligned and should be checked by a professional.
A bald tyre will almost always be incapable of properly dispersing water in wet weather. It is no longer recommended to change the wheels and tyres at regular intervals; this simply means that all five must be replaced at the same time. It is, however, up to you to make the decision.