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 $183.90   3.253%   $189.8823   $189.88   $189.90   $189.90  
  Purchase #2 $20.80   3.370%   $21.501   $21.50   $21.50   $21.50  
  Purchase #3 $183.92   6.321%   $195.5456   $195.55   $195.55   $195.50  
  Purchase #4 $173.39   10.743%   $192.0173   $192.02   $192.00   $192.00  
  Purchase #5 $31.96   8.397%   $34.6437   $34.64   $34.65   $34.60  
  Purchase #6 $6.48   13.820%   $7.3755   $7.38   $7.40   $7.40  
  Purchase #7 $122.98   13.685%   $139.8098   $139.81   $139.80   $139.80  
  7 purchase totals = $723.43       $780.7752   $780.78   $780.80   $780.70