Yep, they can keep sticking for a while and then come down nevertheless, so causin' an irregular level of fuel... BUT, IMHO there's too much worrying about float levels in this forum. If you have the bowls removed, you could check for smooth movement. You could polish the parts a bit. But unless you are sure the level has been tampered with, I wouldn't worry. Sometimes I have the feeling that modern gasoline is - how shall I put it - less 'lubricating' than the old lead containing gasoline. Not that I want that lead back, oh no.