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 $222.44   12.818%   $250.9524   $250.95   $250.95   $251.00  
  Purchase #2 $65.49   6.195%   $69.5471   $69.55   $69.55   $69.50  
  Purchase #3 $161.29   8.216%   $174.5416   $174.54   $174.55   $174.50  
  Purchase #4 $125.80   10.288%   $138.7423   $138.74   $138.75   $138.70  
  Purchase #5 $131.82   8.184%   $142.6081   $142.61   $142.60   $142.60  
  Purchase #6 $110.72   9.165%   $120.8675   $120.87   $120.85   $120.90  
  Purchase #7 $61.57   9.593%   $67.4764   $67.48   $67.50   $67.50  
  7 purchase totals = $879.13       $964.7354   $964.74   $964.75   $964.70