The guides can be helpful, but some of them were created months or years ago and many champions have undergone changes as well as changes to the game since. My advice is to select the masteries you feel are great for your champ.
Example) you might want to use support and offense for a damage dealer that has some important buffs in their kit, for accuracy masteries and TM masteries, then offensive damage bonus masteries. However, if you go defense tree route, you could keep them alive longer with damage mitigation masteries, and counter attack masteries if they have solid A1s.
I know that I have a few support champs where they did no damage at all and ive used offense masteries for them mainly for tier 6 masteries like Warmaster to speed up my dungeon/CB runs. All three are good choices whichever two you choose, and then there are some champions whose masteries will be pretty obvious to use when you start building them.
Make sure you gear them first, to see where the stats come out at in the end, then do masteries. I realize this may not always be the case, and im sure somebody will disagree with atleast one piece of advice ive given, but this is normally how I build champs and while im not a whale am pretty satisfied with my account progress. Hope this helps.
