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 $29.80   3.357%   $30.8004   $30.80   $30.80   $30.80  
  Purchase #2 $154.39   10.645%   $170.8248   $170.82   $170.80   $170.80  
  Purchase #3 $63.88   12.415%   $71.8107   $71.81   $71.80   $71.80  
  Purchase #4 $131.88   13.584%   $149.7946   $149.79   $149.80   $149.80  
  Purchase #5 $123.73   9.227%   $135.1466   $135.15   $135.15   $135.10  
  Purchase #6 $82.92   9.154%   $90.5105   $90.51   $90.50   $90.50  
  Purchase #7 $51.46   11.770%   $57.5168   $57.52   $57.50   $57.50  
  7 purchase totals = $638.06       $706.4044   $706.40   $706.35   $706.30