Is mileage only not reimbursable when driving from home to office and office to home?

I have an office at my company but drive to our 100+ customers for service work with my personal vehicle. I submit mileage and I am reimbursed for it. There seems to be a gray area that I would be responsible for the first and last mileage of the day. I read and reread the IRS guidelines and believe that all mileage is reimbursable except between my main office and home, under the temporary workplace guidelines. This would include driving a shorter distance to a customer site first thing in the morning or if I was at a customer site and drove home at the end of the day, I believe all these miles are reimbursable.