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 $239.87   7.309%   $257.4021   $257.40   $257.40   $257.40  
  Purchase #2 $191.79   10.868%   $212.6337   $212.63   $212.65   $212.60  
  Purchase #3 $115.80   7.417%   $124.3889   $124.39   $124.40   $124.40  
  Purchase #4 $95.76   11.430%   $106.7054   $106.71   $106.70   $106.70  
  Purchase #5 $183.97   5.161%   $193.4647   $193.46   $193.45   $193.50  
  Purchase #6 $11.64   9.345%   $12.7278   $12.73   $12.75   $12.70  
  Purchase #7 $87.96   12.404%   $98.8706   $98.87   $98.85   $98.90  
  7 purchase totals = $926.79       $1,006.1932   $1,006.19   $1,006.20   $1,006.20