Yes, after that it's just a matter of personal preference. I personally do not collect incomplete sets, I do not have the time, patience or interest for chasing missing cards. I also collect newer stuff, 1950s and up mostly, as older 20s to 40s cards do not interest me. Most incomplete sets are pre 50s. Finally, I hate pinholes in cards, so that's something to think about. All collectors are different.
What you could do if you want to stick to modern cards (post 80s) is buy a cheap set no one cares about (therefore original) and compare its backside to all your future purchases.
T
eatbrie is right. Buy a few more cards from different movies in the era you're interested in. You can read about authentication all day online, but nothing beats your own five senses (OK, maybe don't *taste* your lobby cards, but your other senses will tell you a lot).
I am a big lover of lobby cards. It's great to see the images taken by the dedicated stills photographer expressly for promotional purposes. So I was always happy to hunt down individual cards. In fact, when I was younger that's the only way I could afford to buy lobby cards, but it was so fun to cobble together a complete set of lobby cards over time, and that's what this hobby is all about, right? It's collecting for the fun of it.
Condition is important to me, too. But pinholes aren't necessarily a deal-breaker to me, especially if it's a rare title, or if it makes the price better for me. Anyway, there's also that element of authenticity when your poster has signs of being used in a theater. I can live with that, since it's my love for cinema history that got me into this side of collecting.