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 $248.43   3.397%   $256.8692   $256.87   $256.85   $256.90  
  Purchase #2 $247.45   7.458%   $265.9048   $265.90   $265.90   $265.90  
  Purchase #3 $236.77   11.746%   $264.581   $264.58   $264.60   $264.60  
  Purchase #4 $83.78   13.416%   $95.0199   $95.02   $95.00   $95.00  
  Purchase #5 $28.73   6.735%   $30.665   $30.67   $30.65   $30.70  
  Purchase #6 $151.42   6.753%   $161.6454   $161.65   $161.65   $161.60  
  Purchase #7 $58.61   12.212%   $65.7675   $65.77   $65.75   $65.80  
  7 purchase totals = $1,055.19       $1,140.4528   $1,140.46   $1,140.40   $1,140.50