LOTS of discretion.....I change the middle MA with market conditions on my one minute chart all the time.....I change the hours that I trade all the time......and I do not trade if I feel I will get creamed in chop...... I find the best rule is that there is none...
I think this is very dependent upon the individual trader. However, I believe the vast majority of individuals will perform better if they have and follow a strict system based on a clear set of trading principles. I say "principles" because most people unconsciously don't like to follow "rules". I further believe the system should use as few principles as possible and as few data sources as possible. That is, not a lot of charts and a lot of indicators. There is a thread discussing "simple is better?" The best one I've seen uses only one chart, two indicators and a MA. I used to use all kinds of stuff and love to trade, until I lost over 100K, now I love to trade because I make a lot of money and use no stuff!