I answered someone's question regarding rollback segment tablespace fragmentation last night and it got me to reviewing some of the details of my own database. This led to questions for your ponderment
1.) Highwater mark for RBS. I can imagine that this would have some meaning where optimal is not set. However, does it have any practical implications where I use optimal?
2.) Oracle recommends that all extents in a rollback segment be the same size. Yet in their database creation scripts they create RBS with initial 100K, next 250K. Wassupwidat?
3.)Optimal. I have set my optimal to 500 K (512,000 bytes)(initial 100K, next 250K, and currently extent count is 3), yet the RB segments are being autoshrunk to 612352 bytes as shown in DBA_ROLLBACK_SEGS. Not that it really matters, but I wonder why they're not being shrunk to 500K?
Any ideas?

Thanks.