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 $20.67   3.442%   $21.3815   $21.38   $21.40   $21.40  
  Purchase #2 $190.63   10.377%   $210.4117   $210.41   $210.40   $210.40  
  Purchase #3 $109.59   11.846%   $122.572   $122.57   $122.55   $122.60  
  Purchase #4 $148.74   12.172%   $166.8446   $166.84   $166.85   $166.80  
  Purchase #5 $149.36   8.335%   $161.8092   $161.81   $161.80   $161.80  
  Purchase #6 $115.84   13.460%   $131.4321   $131.43   $131.45   $131.40  
  Purchase #7 $49.95   11.612%   $55.7502   $55.75   $55.75   $55.80  
  7 purchase totals = $784.78       $870.2013   $870.19   $870.20   $870.20