To maximize your merits, consider doing task forces. Every week, three task forces / strike forces / trials are designated "weekly strike targets". Upon completion (if you are not yet level 50), you will receive a large amount of bonus XP (up to half a level worth). You will also, regardless of your level, get DOUBLE reward merits. You can receive this bonus once per week per character. Right now, the weeklies are higher level.
Starting Sept 23rd the weekly will be Positron (minimum level 8 for part 1, minimum level 11 for part 2). Two weeks after that, Synapse will be up (min level 15). Two weeks after THAT, Penny Yin (min level 20). Followed at 2 week intervals by Citadel, Manticore and Numina. These six task forces are the 'Signature Task Forces'. I do these on every single character for the massive bonuses and also the Task Force Commander accolade power. This gives your character 5% additional hit points.
Personally, I start slotting select IOs around level 35 and add more as I level up. This reduces the cost of upgrading the SOs, although I have some SOs right until level 50 in the powers where I'm going to put purple recipes.