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 $207.54   12.504%   $233.4908   $233.49   $233.50   $233.50  
  Purchase #2 $17.41   4.794%   $18.2446   $18.24   $18.25   $18.20  
  Purchase #3 $115.83   3.838%   $120.2756   $120.28   $120.30   $120.30  
  Purchase #4 $126.30   7.381%   $135.6222   $135.62   $135.60   $135.60  
  Purchase #5 $115.47   12.296%   $129.6682   $129.67   $129.65   $129.70  
  Purchase #6 $246.69   13.732%   $280.5655   $280.57   $280.55   $280.60  
  Purchase #7 $91.69   13.310%   $103.8939   $103.89   $103.90   $103.90  
  7 purchase totals = $920.93       $1,021.7608   $1,021.76   $1,021.75   $1,021.80