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 $20.51   3.505%   $21.2289   $21.23   $21.25   $21.20  
  Purchase #2 $158.97   7.394%   $170.7242   $170.72   $170.70   $170.70  
  Purchase #3 $30.53   3.396%   $31.5668   $31.57   $31.55   $31.60  
  Purchase #4 $28.36   12.376%   $31.8698   $31.87   $31.85   $31.90  
  Purchase #5 $70.83   8.470%   $76.8293   $76.83   $76.85   $76.80  
  Purchase #6 $154.40   13.206%   $174.7901   $174.79   $174.80   $174.80  
  Purchase #7 $36.87   10.543%   $40.7572   $40.76   $40.75   $40.80  
  7 purchase totals = $500.47       $547.7663   $547.77   $547.75   $547.80