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 $135.44   3.630%   $140.3565   $140.36   $140.35   $140.40  
  Purchase #2 $60.68   11.744%   $67.8063   $67.81   $67.80   $67.80  
  Purchase #3 $63.94   8.391%   $69.3052   $69.31   $69.30   $69.30  
  Purchase #4 $127.77   8.323%   $138.4043   $138.40   $138.40   $138.40  
  Purchase #5 $47.87   6.535%   $50.9983   $51.00   $51.00   $51.00  
  Purchase #6 $167.67   4.437%   $175.1095   $175.11   $175.10   $175.10  
  Purchase #7 $37.41   3.864%   $38.8555   $38.86   $38.85   $38.90  
  7 purchase totals = $640.78       $680.8356   $680.85   $680.80   $680.90