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 $17.28   4.592%   $18.0735   $18.07   $18.05   $18.10  
  Purchase #2 $16.29   5.245%   $17.1444   $17.14   $17.15   $17.10  
  Purchase #3 $77.43   5.747%   $81.8799   $81.88   $81.90   $81.90  
  Purchase #4 $83.25   9.114%   $90.8374   $90.84   $90.85   $90.80  
  Purchase #5 $238.33   8.343%   $258.2139   $258.21   $258.20   $258.20  
  Purchase #6 $224.85   4.595%   $235.1819   $235.18   $235.20   $235.20  
  Purchase #7 $98.43   10.592%   $108.8557   $108.86   $108.85   $108.90  
  7 purchase totals = $755.86       $810.1867   $810.18   $810.20   $810.20