With Tegan, that depends on what you want the story to be with her. If you want the relationship to transfer from being casual hookups to something more, have sex with her during the pledge week. If you want the relationship to move from being friends into something more, wait. There's not a right or wrong way, but obviously having sex with her earlier means more scenes you won't get the opportunity for if you wait.
The Bonnie appears as part of the Bobbie choice, so you've also got to have a good score with Bobbie to open it up. Maybe I should have mentioned that before, my bad. But there'll be a future path that doesn't require a good Bobbie score, the choice in the Jess conversation will just alter Jess' involvement really.
For Imogen if you've got a good score and "Enquire further" at the skatepark you'll get the option to kiss her.