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 $58.98   12.219%   $66.1868   $66.19   $66.20   $66.20  
  Purchase #2 $229.85   5.392%   $242.2435   $242.24   $242.25   $242.20  
  Purchase #3 $33.93   13.473%   $38.5014   $38.50   $38.50   $38.50  
  Purchase #4 $95.28   11.602%   $106.3344   $106.33   $106.35   $106.30  
  Purchase #5 $190.99   10.777%   $211.573   $211.57   $211.55   $211.60  
  Purchase #6 $116.29   10.643%   $128.6667   $128.67   $128.65   $128.70  
  Purchase #7 $131.55   11.288%   $146.3994   $146.40   $146.40   $146.40  
  7 purchase totals = $856.87       $939.9052   $939.90   $939.90   $939.90