I am using automated upload of my pics to Camera Uploads folder in Dropbox, which would be a great service if only I didn't need to go through my photos twice. If I clear the bad ones from my phone the changes don't sync to Dropbox and vice versa. Any way to force these to synchronize better, in a bi-directional way?
Dropbox will not delete files from your mobile device after they are uploaded. The files on your camera, phone, tablet, or SD Card will remain on your device until you remove them manually.
Please read this Dropbox Article.
You capture the images and upload on the dropbox, if you found some unclear pics you can delete it later manually. You can increase the camera syn. time in camera settings.
As Gaurav said, Dropbox won't (in fact can't) delete photos from your device. However, if you want to only go through your photos once, you can turn off background uploading in the Dropbox app settings. Then once you go through your photos and delete the ones you don't want, open the Dropbox app and let the photos sync. However, this loses the feature of having an automatic backup of the photos.