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 $126.83   9.772%   $139.2238   $139.22   $139.20   $139.20  
  Purchase #2 $184.53   10.807%   $204.4722   $204.47   $204.45   $204.50  
  Purchase #3 $53.53   9.649%   $58.6951   $58.70   $58.70   $58.70  
  Purchase #4 $91.83   11.667%   $102.5438   $102.54   $102.55   $102.50  
  Purchase #5 $166.99   5.435%   $176.0659   $176.07   $176.05   $176.10  
  Purchase #6 $176.83   5.593%   $186.7201   $186.72   $186.70   $186.70  
  Purchase #7 $125.61   6.847%   $134.2105   $134.21   $134.20   $134.20  
  7 purchase totals = $926.15       $1,001.9314   $1,001.93   $1,001.85   $1,001.90