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 $17.80   3.208%   $18.371   $18.37   $18.35   $18.40  
  Purchase #2 $13.37   4.484%   $13.9695   $13.97   $13.95   $14.00  
  Purchase #3 $164.59   9.561%   $180.3264   $180.33   $180.35   $180.30  
  Purchase #4 $175.79   6.637%   $187.4572   $187.46   $187.45   $187.50  
  Purchase #5 $151.76   3.707%   $157.3857   $157.39   $157.40   $157.40  
  Purchase #6 $135.79   5.726%   $143.5653   $143.57   $143.55   $143.60  
  Purchase #7 $6.93   13.152%   $7.8414   $7.84   $7.85   $7.80  
  7 purchase totals = $666.03       $708.9165   $708.93   $708.90   $709.00