Depends. You should tip artists who you've worked with and both of you are happy, however tipping an artist you just started working with is a crap shoot, and you won't know the quality until its done.

Historically, I'll tip folks, but I've had enough problems with first time use artists that I refuse to tip the first time I use: one took three months to do a single page piece despite doing other work in between, the other I'd been wanting work from them for years and they did such a terrible job (poor quality, changing the idea completely, doing one less character than paid for), that I won't tip now unless I know them and have worked with them in the past.

Overall, asking for tips isn't a bad thing.

Well, of course tipping is fully optional, but I feel kinda weird having it up if I'm doing commissions.

nah, nothing wrong with asking.

I know, but it feels like double dipping :P

Nah. Its not forcing anything.

Then we shall see how well this works

Optional tipping of visual or textual artists is perfectly ok. The commissioner pays for product and may enjoy privately. If it's uploaded then those who enjoy also may tip. More art exposure, more appreciation of effort. Nobody needs middlemen, managers, companies with their grubby paws out.

Thanks for the input. It still just feels weird to have it up even though I haven't done anything for a specific person.