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 $27.48   12.844%   $31.0095   $31.01   $31.00   $31.00  
  Purchase #2 $174.62   10.259%   $192.5343   $192.53   $192.55   $192.50  
  Purchase #3 $161.98   11.110%   $179.976   $179.98   $180.00   $180.00  
  Purchase #4 $187.74   5.413%   $197.9024   $197.90   $197.90   $197.90  
  Purchase #5 $53.76   13.200%   $60.8563   $60.86   $60.85   $60.90  
  Purchase #6 $185.63   11.619%   $207.1983   $207.20   $207.20   $207.20  
  Purchase #7 $23.76   10.246%   $26.1944   $26.19   $26.20   $26.20  
  7 purchase totals = $814.97       $895.6712   $895.67   $895.70   $895.70