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 $19.95   8.598%   $21.6653   $21.67   $21.65   $21.70  
  Purchase #2 $183.37   4.119%   $190.923   $190.92   $190.90   $190.90  
  Purchase #3 $19.73   11.570%   $22.0128   $22.01   $22.00   $22.00  
  Purchase #4 $171.75   10.124%   $189.138   $189.14   $189.15   $189.10  
  Purchase #5 $42.50   5.820%   $44.9735   $44.97   $44.95   $45.00  
  Purchase #6 $102.85   12.219%   $115.4172   $115.42   $115.40   $115.40  
  Purchase #7 $52.85   5.357%   $55.6812   $55.68   $55.70   $55.70  
  7 purchase totals = $593.00       $639.8110   $639.81   $639.75   $639.80