Yeah, I'd swear it takes into account how much of a pain in the ass it is to get skill up fodder when deciding whether to grant a skill-up or not. My vampire got three skills on about 30 feeds, and I got one naga skill up on ten tries.Cool, after Poring giving me 500 shynee for my Angelion maybe it'll be useful. You know since having a Valk above skill 2 is asking too much! Max'd Duke Vamp, Siren (almost #2 also), Gigas, a few levels on Chu Chu (not near enough for how many I fed him).... and something else I can't remember. Saved up about 25 for next two heroes and a Siegfried.
Meanwhile, I fed two big blue slimes from Poring to my shardran. It's skill level three now.