Re incisions - I think I may have actually put this in a post somewhere on this board before, but will add here just in case. The pad of the palm at the base of your thumb is an easy place for a small incision thats not very painful, and doesnt really scar if you use a scalpel. Its mostly fleshy there, tiny veins only, and fairly easy to "excuse" if you need to - IE you cut yourself while cutting a bagel is a good one, unless you are wheat free like me, LOL.
As to the risks of licking or sucking on the wounds, the smaller the cut it seems the lower the risk, scientifically speaking - if we are talking about a healthy human with no open sores in their mouths, no active gingivitis etc. Saliva actually has nitrite in it which breaks down into nitric oxide, a chemical compound that is effective in protecting cuts and scratches from unwanted bacteria. In addition to nitric oxide, human saliva contains other natural antibacterial agents—namely, lactoferrin, lactoperoxidase, and lysozyme. Saliva also contains histatin, a small protein which also helps kill bacteria and speed healing. However all of these are really only effective in small wounds, like finger stick punctures, paper cuts, and wounds that dont penetrate all layers of the skin.
That said, as to unclean mouths - some vampires will swill with Lysterine or a 100 proof alcohol before commencing feeding directly from a wound. This would kill some of the common bacterias most people worry about, but it also has the potential of stinging like hell when their mouth touches the wound.

If mouth to wound contact is preferable though, and lets face it some donors really really like the intimacy of it, so no matter what we say against it people are going to still do it, then have the vampire swill first. What they should NOT do is brush their teeth before hand. Brushing can open tiny wounds along their gums, and aggravate dormant gingivitis - you definately dont want that.