I tend to agree. You never know how difficult a cleaning job truly will be until you do it the first time! A place may appear sort of clean, but when you dig a bit deeper, you may find out it is not at all clean. I always err on the "dirty side" because often, I am correct.
I do offer new customers a slightly lower rate for the first couple cleanings, and make it clear that if they are pleased with my work, the rate will go up to my local market rate. This gives me the time to find out how difficult any job will be and know how to handle the hourly rate a lot better. People seem to like this plan and few have had difficulty paying my higher rate, once they see what a good job I do!