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 $17.55   7.353%   $18.8405   $18.84   $18.85   $18.80  
  Purchase #2 $27.84   10.672%   $30.8111   $30.81   $30.80   $30.80  
  Purchase #3 $113.99   10.611%   $126.0855   $126.09   $126.10   $126.10  
  Purchase #4 $99.36   11.813%   $111.0974   $111.10   $111.10   $111.10  
  Purchase #5 $80.48   6.788%   $85.943   $85.94   $85.95   $85.90  
  Purchase #6 $144.42   9.125%   $157.5983   $157.60   $157.60   $157.60  
  Purchase #7 $239.56   13.139%   $271.0358   $271.04   $271.05   $271.00  
  7 purchase totals = $723.20       $801.4116   $801.42   $801.45   $801.30