Ok if you here of a sick friend or love one and you say a prayer for them isn't it the same as casting? So to me it seems the same..wouldn't you need their consent the same way you need their consent to do a spell for them?
Praying often involves asking a higher power to do it for you. In such a case I don't think permission is needed because, regardless of what the other person believes, in the praying individual's belief the deity probably won't heal if the injured person would not consent to it. However, it may still be considered polite to ask.
Prayer tends to imply (to a lot of people) that you are leaving the outcome up to some higher force, while "casting" makes it seem like you are the one affecting everything. To me they are ultimately the same, both connecting with a higher force to bring about what you want done. Casting just tends to be more assertive, more direct. I think that the whole "asking for permission" thing is part of the New Age magick movement, very concerned for the interests of others. If it is possible (i.e. you won't sound crazy or scare the person) to ask permission, by all means do so. Praying for someone I wouldn't say requires permission, because again, it's asking for the influence of a deity and is generally more "acceptable" in terms of supernatural influence in an event.
Sorry, kinda babbly -.-
In short: Prayer = no permission, Spell = get permission if you can and use your best judgement.
Praying and casting are the same thing. Most of the time you call on a higher power to aid you in your casting(even if its just calling the corners). Not to mention spells dont have to rime and Amen is translated as So be it.