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 $29.90   3.647%   $30.9905   $30.99   $31.00   $31.00  
  Purchase #2 $114.43   3.263%   $118.1639   $118.16   $118.15   $118.20  
  Purchase #3 $192.41   12.286%   $216.0495   $216.05   $216.05   $216.00  
  Purchase #4 $110.93   7.230%   $118.9502   $118.95   $118.95   $119.00  
  Purchase #5 $232.39   5.404%   $244.9484   $244.95   $244.95   $244.90  
  Purchase #6 $170.27   12.108%   $190.8863   $190.89   $190.90   $190.90  
  Purchase #7 $12.69   13.782%   $14.4389   $14.44   $14.45   $14.40  
  7 purchase totals = $863.02       $934.4277   $934.43   $934.45   $934.40