I agree with most of what you say, however:
System Spd dial - Generally no, there are often used for unique routing (gateway controller resilient config etc) on a basic setup with all users in the same physical location & nothing complex, possibly, or yes with exceptions, E.g. share 10XX but not 11XX)
Call Rerouting 1st, 2nd, always (not sure on this one so please discuss) - Again a difficult one, important & problematic for resiliency. My thoughts are to share all, but configure specific ranges E.g. 1-9 common to all site numbers (networked Vmail or external destinations via the shared speed calls) 10-19 specific for site 1, 20-29 specific to site 2 etc. Requires a lot of planning, but is REALLY important for resiliency
Digit mod, yes, but again I like to see the following:
1-10 are absorb digits 1-10 (E.g. DM 5 is absob 5) therefore on all sites you know that if you want to absorb 0 digits you select 10. Then all other ranges are for specific mods and all are shared.I have changed a seemingly unused DM on a site & screwed it royally.
ARS, not unless all controllers are co-located & even then with reservations. ARS typically does not change much & I prefer to leave it alone.