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 $70.87   11.457%   $78.9896   $78.99   $79.00   $79.00  
  Purchase #2 $87.91   8.227%   $95.1424   $95.14   $95.15   $95.10  
  Purchase #3 $21.60   8.557%   $23.4483   $23.45   $23.45   $23.40  
  Purchase #4 $209.46   6.151%   $222.3439   $222.34   $222.35   $222.30  
  Purchase #5 $45.64   6.444%   $48.581   $48.58   $48.60   $48.60  
  Purchase #6 $39.87   11.812%   $44.5794   $44.58   $44.60   $44.60  
  Purchase #7 $13.65   11.211%   $15.1803   $15.18   $15.20   $15.20  
  7 purchase totals = $489.00       $528.2649   $528.26   $528.35   $528.20