Anytime anyone 'touches' your bags, other than your dh, you should expect to tip at about $1-2 per bag. So....if we have only carryon bags with us, that we have taken on the bus (they are small, so it's permissable) we don't tip. Now, I have had really entertaining bus drivers who I felt deserved a tip because they went above and beyond what I expected them to do. So, that one driver got a $5 bill from me even though I had no bags other than my carryon tote.
Now...I have had rides on DME where the driver has come out and said that tips are always appreciated! That, in addition to the sign at the front of the bus telling me the same thing. Nope, that driver gets nothing from me. I figure that the driver is getting paid to drive the bus...I don't tip him for that. But as I said before, I do tip if the driver has done something extra. And even then, if that driver mentions 'appreciating a tip'....nothing comes out of my pocket.
As far as bell services goes...I tip the person that delivers my bags to me, or takes them away for storage. Same for the person who gets them out of storage for me.