Great article above, I found that 5mm works best for me - not so much for mounts and dismounts but for absorbing the various terrain. Being your first cross race, a 1cm saddle difference will be the least of your concerns. Lower the seat slightly and go out and have fun. After a few races, you'll know better if you need to raise or lower your seat.
1970 Raleigh Tourist, 1972 Raleigh Sports, 1984 Miele Road Bike SS, 1985 Bianchi Nuovo Alloro, Cannondale F400, Opus Avro 29er, Jamis Sonik, Blue CXC, Bianchi San Jose, Cervelo SL-SLC, Cervelo S3 Di2 and a wife that loves me!