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 $58.52   13.351%   $66.333   $66.33   $66.35   $66.30  
  Purchase #2 $111.34   9.475%   $121.8895   $121.89   $121.90   $121.90  
  Purchase #3 $160.26   5.232%   $168.6448   $168.64   $168.65   $168.60  
  Purchase #4 $179.70   11.844%   $200.9837   $200.98   $201.00   $201.00  
  Purchase #5 $40.38   4.241%   $42.0925   $42.09   $42.10   $42.10  
  Purchase #6 $170.85   9.859%   $187.6941   $187.69   $187.70   $187.70  
  Purchase #7 $198.46   7.866%   $214.0709   $214.07   $214.05   $214.10  
  7 purchase totals = $919.51       $1,001.7085   $1,001.69   $1,001.75   $1,001.70