I generally agree with this, but there are some exceptions:
1. For large, repeat clients that have demonstrated loyalty, it's okay to offer a modest discount, no more than 10%. You still cite your rack rate on the invoice, but at the end add a "Loyalty Discount" so they know they are getting this discount for a reason.
2. While you should not reduce your rate, it can be okay to write off your time. If it takes you 20 hours to do something, but you're afraid the client will balk at the bill and run away, rather than lowering your rate, invoice them for 10 hours, or whatever seems reasonable.
3. When you're just starting out, and have limited understanding of the value of your skills compared to the competition, you may not know what your rate should be. In these cases, you may want to consider flat rate arrangements.