Frank and TD said it all - browsing your newly imported images and weeding out the ones you do not want to keep - really is easier done in Aperture than in iPhoto, and the advantage is that you can combine this selection process with the proper tagging and rating of your images at the same time. If you set keywords in iPhoto these will not map properly to a hierarchical keyword structure in Aperture.
And it is risky to delete automatically when you are importing: You may have applied presets that change the images, applied the wrong time zone adjustmens, overwritten the image captions, etc. etc....
Then it is much easier to import again and not to be forced to fix this mess doing it manually, but that will not be possible if you deleted the images automatically before you had a chance to check if the import is complete and correct.
Needless to say, the same goes for automatically deleting from the camera when importing ...
But to answer your question: 🙂
If for some reason your workflow requires you to import first to iPhoto and later from iPhoto to Aperture, then it is useful to treat the intermediate iPhoto libraries like film rolls - one iPhoto Library for each shoot. Import the complete Library into Aperture using File-> Import iPhoto Library,
for if you import selectively from the iPhoto Browser, you will not get proper Version-Master pairs, but two stacked master images for the original master and the edited versions.
So if your iPhoto Library is just one shoot at a time, you can import the complete library and then move this partial Library to your backup location after importing it to Aperture - no need to delete single images in the iPhoto library on import.
I use this method occasionally when I'm roughing it on a hiking or sailing trip, with only one old MacBook (and only iPhoto) to share between my husband and me.
Regards
Léonie