If you have the typical Windows 7 HD setup it will have a 100-200mb System Reserved partition which boots Windows 7, followed by the C partition housing the OS. Then it may also have a data, Recovery and tools partition.
If you reinstall only to C and ignore the other partitions, it will place the System boot files on C abandoning the System Reserved partition for booting Windows 7 and making it wasted space. The only way to avoid this is to delete both System Reserved and C during install so that the installer handles this correctly.
Now as to any other partitions like Data, Recovery or OEM Tools which comes on many factory preinstalled Windows 7 machines. Data is up to you although saving your data there during reinstall is not a responsible backup method. Recovery and OEM Tools (bootable diagnostics) may or may not run afterwards so we recommend making your Recovery disks before reinstall.
This is further explained with more details on the various OEM partitions viability after reinstall in Clean Reinstall - Factory OEM Windows 7
. Many users don't even want Factory Recovery which contains the preinstalled crap that throttles and corrupts Windows 7, so deleting those partitions is fine with them after perhaps making their Recovery disks as a backup in case a new owner might want them in the future.