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 $13.37   3.684%   $13.8626   $13.86   $13.85   $13.90  
  Purchase #2 $12.91   9.190%   $14.0964   $14.10   $14.10   $14.10  
  Purchase #3 $75.89   4.281%   $79.1389   $79.14   $79.15   $79.10  
  Purchase #4 $67.53   11.766%   $75.4756   $75.48   $75.50   $75.50  
  Purchase #5 $187.49   7.628%   $201.7917   $201.79   $201.80   $201.80  
  Purchase #6 $117.41   9.683%   $128.7788   $128.78   $128.80   $128.80  
  Purchase #7 $141.28   10.671%   $156.356   $156.36   $156.35   $156.40  
  7 purchase totals = $615.88       $669.5000   $669.51   $669.55   $669.60