I think this is totally up to you. There is nothing wrong with charging full day fees, you are open and willing to provide full day care.
But, if you aren't overly concerned about the full day income you can charge based on time used. When I first opened I preferred a staggered start with the children (back when we could have five 1yr olds) so I just charged in 1/4 day chunks depending how long they were there each day. The parents were more than grateful. I know they would have paid full day fees without complaining but they did seem to really view my offer of the 1/4 day chunk pricing as a gesture of goodwill that eased their settling in (the child adjusted right away but mom and dad were nervous for a while). I will note, that you need to put in writing that this staggered start, lower price is ONLY for one week, prior to the start of full time care. Make sure there is a clear distinction between this one time, one week special pricing and them paying full price for their spot whether or not they use it from XX day on wards!
I also wouldn't consider anything other than a one week easing in to care plan. Monday 1hr (quick, clean drop off right after all the other children arrive; 1/4 pay), Tuesday they stay 2 hours (1/4 day pay), Wednesday stay until right after lunch (1/2 day pay), Thursday stay for nap but leave right after waking (3/4 day pay) Friday they stay the full day. I have found this useful as they come everyday, day one is so short that they come back the next morning comfortable enough, each day gets a bit longer. I always do drop off right after the other children arrive as there are no extra parents coming in and out to unnerve them and you can give them more attention.
I do not find coming a half or full day once or twice a week here and there before starting care to be helpful at all. Again, I would only consider 1 week of transition staying a bit longer each time.