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 $138.60   6.400%   $147.4704   $147.47   $147.45   $147.50  
  Purchase #2 $53.71   12.659%   $60.5091   $60.51   $60.50   $60.50  
  Purchase #3 $17.61   8.204%   $19.0547   $19.05   $19.05   $19.10  
  Purchase #4 $158.27   11.121%   $175.8712   $175.87   $175.85   $175.90  
  Purchase #5 $152.46   4.616%   $159.4976   $159.50   $159.50   $159.50  
  Purchase #6 $50.70   7.316%   $54.4092   $54.41   $54.40   $54.40  
  Purchase #7 $18.48   4.368%   $19.2872   $19.29   $19.30   $19.30  
  7 purchase totals = $589.83       $636.0994   $636.10   $636.05   $636.20