It’s really very simple. Take minimum flight plan fuel when the weather is good and the stats show you’re likely to be fine (and your alternate is decent).
When it’s bad weather, load it on (I always plan for two gos, a decent alternate which may be further away to give a few options) plus some holding fuel either to hold to make the approach or to give you some thinking time.
Loading extra ‘just because’ isn’t being professional. It really adds up over thousands of sectors. On the flip side, take it when you really need to. An avoidable diversion quickly recoups the costs on the few days where it’s really gone to sh*t. You’re being paid to look at your flight and the big picture.