I don't think skycaps are technically airline employees, but in most larger airports they can check your bags and provide you with your boarding passes. I'm not sure how Logan is set up these days, but at some airports the skycaps will take your bag and put it into the airport luggage system on a conveyor belt behind them. At other airports they load a bunch of bags onto a rolling cart and take them into the airport to a screening area. Some airports still make the individual passenger take their luggage to the screening area personally.
Given that Logan was directly involved in 9/11, it's likely that they have some of the strictest policies. I haven't been there in a few years, however. If someone who has flown out of there recently can answer, that would be best.
Otherwise, I'd suggest getting as much info as possible about your options so that you can make a good decision when you get there. There really are a lot of different factors to consider, including what the lines are like that day and your own personal comfort level.
Oh, I usually tip $2-$3 per bag that I'm checking. I've heard of others who tip more or less. If you've ever had a skycap give you a pass on an overweight bag, however, you've already saved more than even a bigger tip would cost you.