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 $71.37   4.233%   $74.3911   $74.39   $74.40   $74.40  
  Purchase #2 $183.41   4.497%   $191.6579   $191.66   $191.65   $191.70  
  Purchase #3 $105.53   9.216%   $115.2556   $115.26   $115.25   $115.30  
  Purchase #4 $154.58   13.602%   $175.606   $175.61   $175.60   $175.60  
  Purchase #5 $95.66   13.411%   $108.489   $108.49   $108.50   $108.50  
  Purchase #6 $53.51   10.342%   $59.044   $59.04   $59.05   $59.00  
  Purchase #7 $233.81   5.473%   $246.6064   $246.61   $246.60   $246.60  
  7 purchase totals = $897.87       $971.0500   $971.06   $971.05   $971.10