Well it really depends on what sets you're using and what your build goals are. When I was IO'ing my Bots/Traps I went for enough defense to hit the softcap on all three positions plus as much recharge as I could cram in.
As for the level 50/not level 50 issues there are two general strategies most people use. The first group (which includes myself) take the lazy route and slot with max level sets since those are gneerally easier to get. The other group aims for recipes int he level 30-33 range since those allow them to keep thier set bonuses whne exemplering down.
As a final note, depending on your sets for the pet powers HOs are often a slightly better choice than set IOs. The Pet damage sets aren't that great and there are some problems with certain pet powers not using them correctly.
Adeon covered the basics pretty well.
For basic pet slotting when I don't care about secondary abilities of said pet, I generally go blood mandate x2 for the recovery bonus, and either edict x3 for the health, or commanding x3 for the recovery/health. I tend to pick active secondaries for my MMs and always take at least one attack power from the primary, so recovery is very beneficial to my playstyle, and health never hurts either.
Hi Masterminds - I have a questions about IO sets. I'm a level 42 MM with 600,000,000+ $$$ to spend on my build (probably more $ to come) and wanted to know what IO sets I should be considering (both if money is an issue and if money is no problem). Any ideas on what IO sets to go after first (if I should wait until level 50 that is OK)? Thanks in advance!!