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 $69.97   6.141%   $74.2669   $74.27   $74.25   $74.30  
  Purchase #2 $244.46   12.276%   $274.4699   $274.47   $274.45   $274.50  
  Purchase #3 $190.78   13.693%   $216.9035   $216.90   $216.90   $216.90  
  Purchase #4 $250.45   3.317%   $258.7574   $258.76   $258.75   $258.80  
  Purchase #5 $215.65   10.616%   $238.5434   $238.54   $238.55   $238.50  
  Purchase #6 $59.77   5.468%   $63.0382   $63.04   $63.05   $63.00  
  Purchase #7 $176.54   11.574%   $196.9727   $196.97   $196.95   $197.00  
  7 purchase totals = $1,207.62       $1,322.9520   $1,322.95   $1,322.90   $1,323.00