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 $231.35   12.800%   $260.9628   $260.96   $260.95   $261.00  
  Purchase #2 $185.77   7.663%   $200.0056   $200.01   $200.00   $200.00  
  Purchase #3 $68.91   6.386%   $73.3106   $73.31   $73.30   $73.30  
  Purchase #4 $129.45   4.569%   $135.3646   $135.36   $135.35   $135.40  
  Purchase #5 $213.57   6.131%   $226.664   $226.66   $226.65   $226.70  
  Purchase #6 $238.68   11.222%   $265.4647   $265.46   $265.45   $265.50  
  Purchase #7 $95.44   9.345%   $104.3589   $104.36   $104.35   $104.40  
  7 purchase totals = $1,163.17       $1,266.1312   $1,266.12   $1,266.05   $1,266.30