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 $121.75   5.721%   $128.7153   $128.72   $128.70   $128.70  
  Purchase #2 $75.66   7.514%   $81.3451   $81.35   $81.35   $81.30  
  Purchase #3 $185.85   6.337%   $197.6273   $197.63   $197.65   $197.60  
  Purchase #4 $86.70   12.369%   $97.4239   $97.42   $97.40   $97.40  
  Purchase #5 $69.98   4.388%   $73.0507   $73.05   $73.05   $73.10  
  Purchase #6 $36.83   7.309%   $39.5219   $39.52   $39.50   $39.50  
  Purchase #7 $41.80   11.538%   $46.6229   $46.62   $46.60   $46.60  
  7 purchase totals = $618.57       $664.3071   $664.31   $664.25   $664.20