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 $234.84   9.741%   $257.7158   $257.72   $257.70   $257.70  
  Purchase #2 $180.90   13.374%   $205.0936   $205.09   $205.10   $205.10  
  Purchase #3 $35.71   13.313%   $40.4641   $40.46   $40.45   $40.50  
  Purchase #4 $69.84   4.430%   $72.9339   $72.93   $72.95   $72.90  
  Purchase #5 $115.89   6.572%   $123.5063   $123.51   $123.50   $123.50  
  Purchase #6 $175.66   7.726%   $189.2315   $189.23   $189.25   $189.20  
  Purchase #7 $130.72   13.162%   $147.9254   $147.93   $147.95   $147.90  
  7 purchase totals = $943.56       $1,036.8706   $1,036.87   $1,036.90   $1,036.80