For the purpose of being jolly, happy, and content this is a very good rule. It is certainly no fun being around a pessimist. So, in general, I think this is a very good rule.
However, for the purpose of understanding the Theory of Everything, I think this is a very bad rule. To assume the best thing, will cloud your judgement, placing greater value on the evidence indicating goodness and lesser value on the evidence indicating badness. To want a result, or to hope for a certain result makes us see things that may not be there, or interpret evidence with a slant towards our hope. I propose a better rule, "Let us seek the truth and follow the evidence wherever that may lead, and then have the courage to believe it".
This is like going to the Doctor and the doctor discovers you have terminal cancer with no hope for recovery. Does the doctor tell you the happy news, that you will live for another 20 years? Or does the doctor tell you the truth, that you will be dead within a month. Do you want the best news or the truth. I want the truth and the courage to believe it. Of course, the truth is sometimes equivalent to the best news. See, I'm an optimist afterall
