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 $249.84   9.678%   $274.0195   $274.02   $274.00   $274.00  
  Purchase #2 $233.87   8.448%   $253.6273   $253.63   $253.65   $253.60  
  Purchase #3 $161.71   6.187%   $171.715   $171.72   $171.70   $171.70  
  Purchase #4 $173.98   11.199%   $193.464   $193.46   $193.45   $193.50  
  Purchase #5 $121.44   11.407%   $135.2927   $135.29   $135.30   $135.30  
  Purchase #6 $36.70   3.222%   $37.8825   $37.88   $37.90   $37.90  
  Purchase #7 $109.42   5.114%   $115.0157   $115.02   $115.00   $115.00  
  7 purchase totals = $1,086.96       $1,181.0167   $1,181.02   $1,181.00   $1,181.00