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 $17.55   3.168%   $18.106   $18.11   $18.10   $18.10  
  Purchase #2 $139.51   9.524%   $152.7969   $152.80   $152.80   $152.80  
  Purchase #3 $211.87   9.750%   $232.5273   $232.53   $232.55   $232.50  
  Purchase #4 $168.63   8.416%   $182.8219   $182.82   $182.80   $182.80  
  Purchase #5 $126.68   9.478%   $138.6867   $138.69   $138.70   $138.70  
  Purchase #6 $111.32   4.406%   $116.2248   $116.22   $116.20   $116.20  
  Purchase #7 $133.87   10.719%   $148.2195   $148.22   $148.20   $148.20  
  7 purchase totals = $909.43       $989.3831   $989.39   $989.35   $989.30