It's always best to make an appointment just in case it is a day when they are going to be overloaded. However I have never heard of them ever turning anyone away.
I actually went in and did my paperwork 2 days before my drop off to ensure that it was no hassle when I dropped the car off and caught my flight. The day I went in was one day ahead of my reservation. I did email them though and they said come on in.
2014 Porsche 911 Turbo S
2015 Mercedes-Benz C300 Sport 4MATIC