As per my knowledge, the ledger is not required for IT depreciation area since this area will b used only for reporting purpose.
Yes, as you said, the depreciation reporting happens at Group assets level / Block of assets level but not at single asset.
You can use the existing asset classes and need not to create any new asset class for this Group asset - IT dep purpose.
If the ledgers which you are assigning is for real time postings, then you may have to specify the account determination.
Please elaborate your requirement of Ledgers.
Thanks,
NSK