It depends on your disks and needs:
- If you always procure the same models (say, with long-term support contracts or by choosing yourself), it does not matter much and you can save yourself the hassle of partitioning. The same applies if your size needs are low, so you replace your 500GB with the currently lowest offer of 1 TB without resizing the pool.
- If you have a variety of different disks from different sources, need to replace with what's available now and/or need to accept slightly inferior conditions (some sectors are destroyed, but the drive functions well and you have enough redundancy anyway), you are more flexible and future-proof with removing a few megabytes from each drive and creating equally-sized partitions.
My personal recommendation:
If you can manage/automate the partitioning step so that it is no additional burden (might already be done by the operating system) when replacing drives, go for the single slightly reduced partition. If money is no objective, you don't need to, because you then just order the exact same drive or a drive with 1TB more of space.