Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $247.27   9.819%   $271.5494   $271.55   $271.55   $271.50  
  Purchase #2 $234.96   13.610%   $266.9381   $266.94   $266.95   $266.90  
  Purchase #3 $182.82   12.239%   $205.1953   $205.20   $205.20   $205.20  
  Purchase #4 $56.36   8.708%   $61.2678   $61.27   $61.25   $61.30  
  Purchase #5 $123.32   3.378%   $127.4857   $127.49   $127.50   $127.50  
  Purchase #6 $43.92   12.871%   $49.5729   $49.57   $49.55   $49.60  
  Purchase #7 $30.47   8.536%   $33.0709   $33.07   $33.05   $33.10  
  7 purchase totals = $919.12       $1,015.0801   $1,015.09   $1,015.05   $1,015.10