Things used to be so much easier when you could attach a message to EG's that you share. I'm not sure why they decided to remove it.
When it comes to last hits, there's really no need to worry about who gets it, except if there's someone who has hit multiple times for millions of SED, in which case you might want to refrain from attacking or consider waiting a little bit if a poke from you might kill it, out of courtesy.
Anyway, if you have a decent friends list, there's usually no reason to use more than a weak seduction per SEG. Unless you're new and highly active or are aiming for the top ranks, getting SEG kills takes more effort than reaching their ranking goal, you'll likely have more SEGs than your regen can keep up with. And soloing a few high hp SEGs usually does the trick for top 1k ranking purposes.
The only time I got a little annoyed when someone KS'd from me was when I used multiple full seduction on a friend's escaping SEG that had no heavy hitters on it, only for someone to do a tiny second weak seduction on it in the 5 seconds it took for me to use a pudding and attack (after I refilled and clicked attack...). I would usually try to make sure my last hit is a full seduction if I want the final hit bonus, but at the time it didn't seem like a good idea to waste 2 focus on the off chance that someone would kill the SEG in those few seconds. Had it been someone's first attack, or a second attack that happened simultaneously, I wouldn't blame them, but the descision and timing in this case was kinda ridiculous :P