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 $19.80   4.609%   $20.7126   $20.71   $20.70   $20.70  
  Purchase #2 $136.36   12.783%   $153.7909   $153.79   $153.80   $153.80  
  Purchase #3 $29.59   9.598%   $32.43   $32.43   $32.45   $32.40  
  Purchase #4 $156.38   12.303%   $175.6194   $175.62   $175.60   $175.60  
  Purchase #5 $8.40   5.371%   $8.8512   $8.85   $8.85   $8.90  
  Purchase #6 $122.35   7.483%   $131.5055   $131.51   $131.50   $131.50  
  Purchase #7 $130.93   4.437%   $136.7394   $136.74   $136.75   $136.70  
  7 purchase totals = $603.81       $659.6490   $659.65   $659.65   $659.60