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 $170.41   11.434%   $189.8947   $189.89   $189.90   $189.90  
  Purchase #2 $242.84   7.868%   $261.9467   $261.95   $261.95   $261.90  
  Purchase #3 $108.79   13.685%   $123.6779   $123.68   $123.70   $123.70  
  Purchase #4 $91.84   12.299%   $103.1354   $103.14   $103.15   $103.10  
  Purchase #5 $124.49   5.352%   $131.1527   $131.15   $131.15   $131.20  
  Purchase #6 $7.71   3.162%   $7.9538   $7.95   $7.95   $8.00  
  Purchase #7 $199.47   10.594%   $220.6019   $220.60   $220.60   $220.60  
  7 purchase totals = $945.55       $1,038.3631   $1,038.36   $1,038.40   $1,038.40