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 $235.96   6.361%   $250.9694   $250.97   $250.95   $251.00  
  Purchase #2 $106.81   3.328%   $110.3646   $110.36   $110.35   $110.40  
  Purchase #3 $44.53   9.182%   $48.6187   $48.62   $48.60   $48.60  
  Purchase #4 $170.89   13.328%   $193.6662   $193.67   $193.65   $193.70  
  Purchase #5 $34.79   6.759%   $37.1415   $37.14   $37.15   $37.10  
  Purchase #6 $203.48   5.615%   $214.9054   $214.91   $214.90   $214.90  
  Purchase #7 $18.45   12.384%   $20.7348   $20.73   $20.75   $20.70  
  7 purchase totals = $814.91       $876.4006   $876.40   $876.35   $876.40