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 $117.33   3.106%   $120.9743   $120.97   $120.95   $121.00  
  Purchase #2 $190.50   9.172%   $207.9727   $207.97   $207.95   $208.00  
  Purchase #3 $125.32   10.292%   $138.2179   $138.22   $138.20   $138.20  
  Purchase #4 $191.90   8.770%   $208.7296   $208.73   $208.75   $208.70  
  Purchase #5 $38.76   12.681%   $43.6752   $43.68   $43.70   $43.70  
  Purchase #6 $173.65   4.429%   $181.341   $181.34   $181.35   $181.30  
  Purchase #7 $114.26   5.274%   $120.2861   $120.29   $120.30   $120.30  
  7 purchase totals = $951.72       $1,021.1968   $1,021.20   $1,021.20   $1,021.20