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 $83.73   7.767%   $90.2333   $90.23   $90.25   $90.20  
  Purchase #2 $54.76   4.533%   $57.2423   $57.24   $57.25   $57.20  
  Purchase #3 $75.30   12.121%   $84.4271   $84.43   $84.45   $84.40  
  Purchase #4 $83.53   5.846%   $88.4132   $88.41   $88.40   $88.40  
  Purchase #5 $222.58   4.623%   $232.8699   $232.87   $232.85   $232.90  
  Purchase #6 $103.54   13.331%   $117.3429   $117.34   $117.35   $117.30  
  Purchase #7 $152.27   4.238%   $158.7232   $158.72   $158.70   $158.70  
  7 purchase totals = $775.71       $829.2519   $829.24   $829.25   $829.10