Sorry for bringing something different to attention but did you tried Custom Calculations for rounding?
You can try something like that.
Setup a custom calculation as described here
use this expression as script.
result = 0.05 - ((Ticket.PSum + Ticket.PreSum + Ticket.Tax) % 0.05)
As custom calculations works differently from regular calculations creating an automatic selector may create issues. Instead you can try enabling it with Update Ticket Calculation action after first item added to ticket. (eg. ticket state changes from new to new orders).
Might be a nice sample for implementing some unusual calculations.