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 $226.28   12.156%   $253.7866   $253.79   $253.80   $253.80  
  Purchase #2 $114.87   8.571%   $124.7155   $124.72   $124.70   $124.70  
  Purchase #3 $110.73   13.347%   $125.5091   $125.51   $125.50   $125.50  
  Purchase #4 $25.45   3.301%   $26.2901   $26.29   $26.30   $26.30  
  Purchase #5 $174.93   8.364%   $189.5611   $189.56   $189.55   $189.60  
  Purchase #6 $56.79   4.428%   $59.3047   $59.30   $59.30   $59.30  
  Purchase #7 $15.76   12.314%   $17.7007   $17.70   $17.70   $17.70  
  7 purchase totals = $724.81       $796.8678   $796.87   $796.85   $796.90