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 $15.40   9.129%   $16.8059   $16.81   $16.80   $16.80  
  Purchase #2 $75.40   4.296%   $78.6392   $78.64   $78.65   $78.60  
  Purchase #3 $121.84   12.557%   $137.1394   $137.14   $137.15   $137.10  
  Purchase #4 $163.88   10.632%   $181.3037   $181.30   $181.30   $181.30  
  Purchase #5 $54.35   8.749%   $59.1051   $59.11   $59.10   $59.10  
  Purchase #6 $182.44   10.367%   $201.3536   $201.35   $201.35   $201.40  
  Purchase #7 $8.77   7.347%   $9.4143   $9.41   $9.40   $9.40  
  7 purchase totals = $622.08       $683.7612   $683.76   $683.75   $683.70