I don't think your partition vs drive question relates to your array being a MD RAID 10. The partition vs drive question is preferential. And when it comes to growing or shrinking arrays, if MDADM can do something for RAID 1, it can probably do it for raid 10.
On that note, it is easier if you do not use the whole drive for your array just in case another drive you get in the future ends up being smaller, and whether or not you address such an issue now or later is a matter of preference. I personally use a partition that is slightly smaller for my MD RAID 10 array because it is safer than messing with sizes after the array is in place.
And in contrast to what Hennes has said, you DO get a performance boost when you use a software RAID 10 on two drives. My performance literally doubled the moment I rebuilt my array into 10 from 1 using the same drives in the same computer. Seems ridiculous RAID 1 doesn't naturally read from both discs, but it doesn't! I am thinking about adding a third drive for double redundancy since I am using older drives and I think I run a high risk of failure during a rebuild due to using older hardware in general. And the speed boost doesn't hurt.
As a note, I even have two different size drives (1TB and 2TB),from 2 different manufacturers. Not the best scenario, but it works great! Writes are slower than a single drive, but reads are fast! Only 2 drives and rebuilds are quick! (no parity) Enjoy your two drive RAID 10 and just enjoy all those people like Hennes who are SURE you need 4 drives. :)