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 $87.34   5.548%   $92.1856   $92.19   $92.20   $92.20  
  Purchase #2 $85.76   10.669%   $94.9097   $94.91   $94.90   $94.90  
  Purchase #3 $165.87   9.366%   $181.4054   $181.41   $181.40   $181.40  
  Purchase #4 $240.86   5.816%   $254.8684   $254.87   $254.85   $254.90  
  Purchase #5 $6.85   5.197%   $7.206   $7.21   $7.20   $7.20  
  Purchase #6 $164.94   9.676%   $180.8996   $180.90   $180.90   $180.90  
  Purchase #7 $25.37   8.437%   $27.5105   $27.51   $27.50   $27.50  
  7 purchase totals = $776.99       $838.9852   $839.00   $838.95   $839.00