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 $15.99   12.259%   $17.9502   $17.95   $17.95   $18.00  
  Purchase #2 $164.62   11.411%   $183.4048   $183.40   $183.40   $183.40  
  Purchase #3 $218.79   6.153%   $232.2521   $232.25   $232.25   $232.30  
  Purchase #4 $131.90   11.314%   $146.8232   $146.82   $146.80   $146.80  
  Purchase #5 $245.60   7.752%   $264.6389   $264.64   $264.65   $264.60  
  Purchase #6 $123.51   11.727%   $137.994   $137.99   $138.00   $138.00  
  Purchase #7 $166.50   3.212%   $171.848   $171.85   $171.85   $171.80  
  7 purchase totals = $1,066.91       $1,154.9112   $1,154.90   $1,154.90   $1,154.90