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 $7.89   13.149%   $8.9275   $8.93   $8.95   $8.90  
  Purchase #2 $12.35   8.204%   $13.3632   $13.36   $13.35   $13.40  
  Purchase #3 $33.96   3.411%   $35.1184   $35.12   $35.10   $35.10  
  Purchase #4 $99.40   3.438%   $102.8174   $102.82   $102.80   $102.80  
  Purchase #5 $122.45   10.244%   $134.9938   $134.99   $135.00   $135.00  
  Purchase #6 $150.75   6.260%   $160.187   $160.19   $160.20   $160.20  
  Purchase #7 $30.39   10.544%   $33.5943   $33.59   $33.60   $33.60  
  7 purchase totals = $457.19       $489.0016   $489.00   $489.00   $489.00