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 $38.27   4.418%   $39.9608   $39.96   $39.95   $40.00  
  Purchase #2 $156.34   8.636%   $169.8415   $169.84   $169.85   $169.80  
  Purchase #3 $172.43   8.333%   $186.7986   $186.80   $186.80   $186.80  
  Purchase #4 $176.34   5.497%   $186.0334   $186.03   $186.05   $186.00  
  Purchase #5 $134.70   8.172%   $145.7077   $145.71   $145.70   $145.70  
  Purchase #6 $164.72   12.308%   $184.9937   $184.99   $185.00   $185.00  
  Purchase #7 $214.29   8.445%   $232.3868   $232.39   $232.40   $232.40  
  7 purchase totals = $1,057.09       $1,145.7225   $1,145.72   $1,145.75   $1,145.70