yeah when I ran D&D I didnt award XP either. keeping track of xp was too much of a pain.
instead i used milestones to level up players. they would level up when they accomplished something big. the milestones could either be personal milestones or group milestones.
this is actually something I started doing when I ran the starwars rpg and ive done it in every rpg ive ran ever since. I give the players milestone rewards when they complete scenarios or campaign arcs or when I feel their characters have had an apotheosis of sorts. And I try to work in character side arcs for each of the players that focus solely on their character to help give them those apotheosis moments.
It bothers your players to have left over exp on their sheet?
yes because they cant spend it lol. theres no way to spend less than 5xp in the alien rpg.
I suppose it wouldnt be an issue if you end them on a number divisible by 5 every other session or so (like if you awarded 7xp one session then 8xp another session which comes out to 15xp). But they dont like having a remainder of XP they cant spend that carries over for several whole sessions.
Although Im considering adding certifications as an additional way to spend XP. Players would have to get certifications to use weapons/equipment thats not on their class list. A certification would probably cost 3xp. That would help with that problem by letting them spend XP in smaller increments.