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 $38.70   4.337%   $40.3784   $40.38   $40.40   $40.40  
  Purchase #2 $38.89   3.870%   $40.395   $40.40   $40.40   $40.40  
  Purchase #3 $84.25   10.357%   $92.9758   $92.98   $93.00   $93.00  
  Purchase #4 $164.69   9.363%   $180.1099   $180.11   $180.10   $180.10  
  Purchase #5 $97.44   11.304%   $108.4546   $108.45   $108.45   $108.50  
  Purchase #6 $73.60   13.251%   $83.3527   $83.35   $83.35   $83.40  
  Purchase #7 $107.27   5.287%   $112.9414   $112.94   $112.95   $112.90  
  7 purchase totals = $604.84       $658.6078   $658.61   $658.65   $658.70