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 $19.87   8.560%   $21.5709   $21.57   $21.55   $21.60  
  Purchase #2 $123.47   7.730%   $133.0142   $133.01   $133.00   $133.00  
  Purchase #3 $123.97   10.189%   $136.6013   $136.60   $136.60   $136.60  
  Purchase #4 $196.26   11.116%   $218.0763   $218.08   $218.10   $218.10  
  Purchase #5 $138.78   13.283%   $157.2141   $157.21   $157.20   $157.20  
  Purchase #6 $127.84   10.154%   $140.8209   $140.82   $140.80   $140.80  
  Purchase #7 $160.47   13.773%   $182.5715   $182.57   $182.55   $182.60  
  7 purchase totals = $890.66       $989.8692   $989.86   $989.80   $989.90