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 $79.91   12.619%   $89.9938   $89.99   $90.00   $90.00  
  Purchase #2 $72.74   10.224%   $80.1769   $80.18   $80.20   $80.20  
  Purchase #3 $19.91   8.347%   $21.5719   $21.57   $21.55   $21.60  
  Purchase #4 $92.70   13.719%   $105.4175   $105.42   $105.40   $105.40  
  Purchase #5 $160.62   9.561%   $175.9769   $175.98   $176.00   $176.00  
  Purchase #6 $225.47   6.422%   $239.9497   $239.95   $239.95   $239.90  
  Purchase #7 $9.49   12.104%   $10.6387   $10.64   $10.65   $10.60  
  7 purchase totals = $660.84       $723.7254   $723.73   $723.75   $723.70