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 $29.63   6.382%   $31.521   $31.52   $31.50   $31.50  
  Purchase #2 $80.95   10.327%   $89.3097   $89.31   $89.30   $89.30  
  Purchase #3 $135.32   11.158%   $150.419   $150.42   $150.40   $150.40  
  Purchase #4 $8.73   3.199%   $9.0093   $9.01   $9.00   $9.00  
  Purchase #5 $148.37   5.847%   $157.0452   $157.05   $157.05   $157.00  
  Purchase #6 $126.78   9.255%   $138.5135   $138.51   $138.50   $138.50  
  Purchase #7 $132.42   12.442%   $148.8957   $148.90   $148.90   $148.90  
  7 purchase totals = $662.20       $724.7134   $724.72   $724.65   $724.60