Are you having a little bit of trouble choosing between a sheers and opaques? Read the remainder of my blog entry for my advice.
What you wear depends on what you need.
If you are wearing it for warmth, and the weather is quite nice out, then you could probably wear sheers.
If you wear it to keep your clothing clean, so that you do not have to dry clean as often, then you ought to wear the most opaque pair that you can get your hands on, without making the pants uncomfortable.
If you wear it as part of a uniform [e.g. security guard; hospital staff; school; cafeteria] there is a bit of flexibility there. Generally speaking, higher level employees should dress more formally, and wear day sheers. That being said, opaques have become very popular, so even wearing opaques with formal clothing is very acceptable too. The only thing is that opaques tend to be a little bit less classic.