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 $246.84   5.302%   $259.9275   $259.93   $259.95   $259.90  
  Purchase #2 $43.99   11.422%   $49.0145   $49.01   $49.00   $49.00  
  Purchase #3 $28.59   12.750%   $32.2352   $32.24   $32.25   $32.20  
  Purchase #4 $163.65   13.328%   $185.4613   $185.46   $185.45   $185.50  
  Purchase #5 $75.62   9.297%   $82.6504   $82.65   $82.65   $82.70  
  Purchase #6 $80.74   5.590%   $85.2534   $85.25   $85.25   $85.30  
  Purchase #7 $76.51   11.102%   $85.0041   $85.00   $85.00   $85.00  
  7 purchase totals = $715.94       $779.5464   $779.54   $779.55   $779.60