Your Brand is the outcome of trust---trust built in you and your services by those you have worked with as well as those that have only heard of you. When you refer someone you need to know how trustworthy they are because if they mess up the person who trusted you is unhappy.
The best way to deal with this is to RECOMMEND people that you personally have worked with and believe provide excellent service.
Anyone that you don't have personal experience with you can REFER with this caveat, "I hear they are good, but I've never worked with them personally. Let me know what you think of them." That gets you off the hook while giving you an excellent way to get a ranking. Most people will tell you about a bad experience without prompting. If the interaction was well above expectations, you'll hear as well.