If you dont choose a perk by the time you would get a new one, you lose it.Hit level 6 without choosing the level 3 perk - you lose the level 3 perk.
Boon is correct. Perks don't stack up waiting for you to choose them. Unused Skill Points, however, will add up between levels.
Thank you Honest Abe, you are a real helper.By critical build do you mean sniper?I have a fast shot 20% crit sg farmer On the same topic, with what build do you farm vc patrols?