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 $12.37   6.859%   $13.2185   $13.22   $13.20   $13.20  
  Purchase #2 $43.44   3.594%   $45.0012   $45.00   $45.00   $45.00  
  Purchase #3 $14.96   11.582%   $16.6927   $16.69   $16.70   $16.70  
  Purchase #4 $197.50   9.349%   $215.9643   $215.96   $215.95   $216.00  
  Purchase #5 $6.60   11.627%   $7.3674   $7.37   $7.35   $7.40  
  Purchase #6 $127.70   6.612%   $136.1435   $136.14   $136.15   $136.10  
  Purchase #7 $181.45   9.501%   $198.6896   $198.69   $198.70   $198.70  
  7 purchase totals = $584.02       $633.0772   $633.07   $633.05   $633.10