If its in the comments, 90% of people won't ever notice it. Most people just read the artist comments, because generally that's where that would go. The artist should really add that to their rules. 
Not everyone has a dA, so not everyone can ask. And not everyone wants to. Plus, some artists no longer use their accounts or don't like responding to strangers and stuff like that.
If it doesn't say you can't do it in the rules and isn't something bad (like you're profiting from their lineart without permission), its generally seen as fine.

Not everyone has a dA, so not everyone can ask. And not everyone wants to. Plus, some artists no longer use their accounts or don't like responding to strangers and stuff like that.
If it doesn't say you can't do it in the rules and isn't something bad (like you're profiting from their lineart without permission), its generally seen as fine.