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 $223.63   11.577%   $249.5196   $249.52   $249.50   $249.50  
  Purchase #2 $195.89   9.557%   $214.6112   $214.61   $214.60   $214.60  
  Purchase #3 $124.47   13.176%   $140.8702   $140.87   $140.85   $140.90  
  Purchase #4 $102.87   5.185%   $108.2038   $108.20   $108.20   $108.20  
  Purchase #5 $218.69   12.654%   $246.363   $246.36   $246.35   $246.40  
  Purchase #6 $233.73   6.275%   $248.3966   $248.40   $248.40   $248.40  
  Purchase #7 $59.28   9.601%   $64.9715   $64.97   $64.95   $65.00  
  7 purchase totals = $1,158.56       $1,272.9359   $1,272.93   $1,272.85   $1,273.00