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 $7.83   4.318%   $8.1681   $8.17   $8.15   $8.20  
  Purchase #2 $109.70   5.805%   $116.0681   $116.07   $116.05   $116.10  
  Purchase #3 $223.81   11.547%   $249.6533   $249.65   $249.65   $249.70  
  Purchase #4 $57.28   10.248%   $63.1501   $63.15   $63.15   $63.20  
  Purchase #5 $41.84   10.792%   $46.3554   $46.36   $46.35   $46.40  
  Purchase #6 $7.60   5.586%   $8.0245   $8.02   $8.00   $8.00  
  Purchase #7 $137.70   9.186%   $150.3491   $150.35   $150.35   $150.30  
  7 purchase totals = $585.76       $641.7686   $641.77   $641.70   $641.90