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 $17.26   7.164%   $18.4965   $18.50   $18.50   $18.50  
  Purchase #2 $191.45   13.863%   $217.9907   $217.99   $218.00   $218.00  
  Purchase #3 $135.57   13.630%   $154.0482   $154.05   $154.05   $154.00  
  Purchase #4 $170.50   6.215%   $181.0966   $181.10   $181.10   $181.10  
  Purchase #5 $225.43   13.423%   $255.6895   $255.69   $255.70   $255.70  
  Purchase #6 $5.29   7.301%   $5.6762   $5.68   $5.70   $5.70  
  Purchase #7 $126.31   3.650%   $130.9203   $130.92   $130.90   $130.90  
  7 purchase totals = $871.81       $963.9180   $963.93   $963.95   $963.90