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 $19.56   5.165%   $20.5703   $20.57   $20.55   $20.60  
  Purchase #2 $161.84   10.432%   $178.7231   $178.72   $178.70   $178.70  
  Purchase #3 $98.62   10.122%   $108.6023   $108.60   $108.60   $108.60  
  Purchase #4 $130.85   10.250%   $144.2621   $144.26   $144.25   $144.30  
  Purchase #5 $23.50   9.239%   $25.6712   $25.67   $25.65   $25.70  
  Purchase #6 $81.53   6.626%   $86.9322   $86.93   $86.95   $86.90  
  Purchase #7 $63.99   3.371%   $66.1471   $66.15   $66.15   $66.10  
  7 purchase totals = $579.89       $630.9083   $630.90   $630.85   $630.90