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 $144.71   9.803%   $158.8959   $158.90   $158.90   $158.90  
  Purchase #2 $20.45   11.658%   $22.8341   $22.83   $22.85   $22.80  
  Purchase #3 $28.78   6.225%   $30.5716   $30.57   $30.55   $30.60  
  Purchase #4 $191.65   11.385%   $213.4694   $213.47   $213.45   $213.50  
  Purchase #5 $15.67   11.332%   $17.4457   $17.45   $17.45   $17.40  
  Purchase #6 $50.34   10.365%   $55.5577   $55.56   $55.55   $55.60  
  Purchase #7 $145.76   9.639%   $159.8098   $159.81   $159.80   $159.80  
  7 purchase totals = $597.36       $658.5842   $658.59   $658.55   $658.60