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 $100.77   8.210%   $109.0432   $109.04   $109.05   $109.00  
  Purchase #2 $18.39   7.529%   $19.7746   $19.77   $19.75   $19.80  
  Purchase #3 $106.89   8.325%   $115.7886   $115.79   $115.80   $115.80  
  Purchase #4 $10.84   3.270%   $11.1945   $11.19   $11.20   $11.20  
  Purchase #5 $239.63   4.442%   $250.2744   $250.27   $250.25   $250.30  
  Purchase #6 $93.25   4.247%   $97.2103   $97.21   $97.20   $97.20  
  Purchase #7 $35.91   6.822%   $38.3598   $38.36   $38.35   $38.40  
  7 purchase totals = $605.68       $641.6454   $641.63   $641.60   $641.70