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 $242.49   6.835%   $259.0642   $259.06   $259.05   $259.10  
  Purchase #2 $105.63   4.451%   $110.3316   $110.33   $110.35   $110.30  
  Purchase #3 $107.38   7.724%   $115.674   $115.67   $115.65   $115.70  
  Purchase #4 $167.40   4.845%   $175.5105   $175.51   $175.50   $175.50  
  Purchase #5 $134.73   6.195%   $143.0765   $143.08   $143.10   $143.10  
  Purchase #6 $25.43   4.655%   $26.6138   $26.61   $26.60   $26.60  
  Purchase #7 $189.63   8.414%   $205.5855   $205.59   $205.60   $205.60  
  7 purchase totals = $972.69       $1,035.8561   $1,035.85   $1,035.85   $1,035.90