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 $8.73   13.153%   $9.8783   $9.88   $9.90   $9.90  
  Purchase #2 $155.87   8.356%   $168.8945   $168.89   $168.90   $168.90  
  Purchase #3 $108.72   10.299%   $119.9171   $119.92   $119.90   $119.90  
  Purchase #4 $205.53   12.232%   $230.6704   $230.67   $230.65   $230.70  
  Purchase #5 $9.62   8.319%   $10.4203   $10.42   $10.40   $10.40  
  Purchase #6 $18.41   13.388%   $20.8747   $20.87   $20.85   $20.90  
  Purchase #7 $78.57   6.302%   $83.5215   $83.52   $83.50   $83.50  
  7 purchase totals = $585.45       $644.1768   $644.17   $644.10   $644.20