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 $110.98   12.548%   $124.9058   $124.91   $124.90   $124.90  
  Purchase #2 $228.31   6.275%   $242.6365   $242.64   $242.65   $242.60  
  Purchase #3 $148.33   13.324%   $168.0935   $168.09   $168.10   $168.10  
  Purchase #4 $152.81   9.141%   $166.7784   $166.78   $166.80   $166.80  
  Purchase #5 $48.34   5.856%   $51.1708   $51.17   $51.15   $51.20  
  Purchase #6 $204.54   3.138%   $210.9585   $210.96   $210.95   $211.00  
  Purchase #7 $8.52   10.242%   $9.3926   $9.39   $9.40   $9.40  
  7 purchase totals = $901.83       $973.9361   $973.94   $973.95   $974.00