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 $9.29   6.412%   $9.8857   $9.89   $9.90   $9.90  
  Purchase #2 $46.74   4.356%   $48.776   $48.78   $48.80   $48.80  
  Purchase #3 $35.75   10.436%   $39.4809   $39.48   $39.50   $39.50  
  Purchase #4 $134.66   10.495%   $148.7926   $148.79   $148.80   $148.80  
  Purchase #5 $201.39   7.290%   $216.0713   $216.07   $216.05   $216.10  
  Purchase #6 $184.68   10.637%   $204.3244   $204.32   $204.30   $204.30  
  Purchase #7 $16.63   8.436%   $18.0329   $18.03   $18.05   $18.00  
  7 purchase totals = $629.14       $685.3638   $685.36   $685.40   $685.40