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 $74.94   12.746%   $84.4919   $84.49   $84.50   $84.50  
  Purchase #2 $161.94   5.564%   $170.9503   $170.95   $170.95   $171.00  
  Purchase #3 $114.66   7.754%   $123.5507   $123.55   $123.55   $123.60  
  Purchase #4 $188.90   6.859%   $201.8567   $201.86   $201.85   $201.90  
  Purchase #5 $119.26   4.280%   $124.3643   $124.36   $124.35   $124.40  
  Purchase #6 $108.50   8.808%   $118.0567   $118.06   $118.05   $118.10  
  Purchase #7 $51.51   3.522%   $53.3242   $53.32   $53.30   $53.30  
  7 purchase totals = $819.71       $876.5948   $876.59   $876.55   $876.80