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 $49.62   13.833%   $56.4839   $56.48   $56.50   $56.50  
  Purchase #2 $16.60   12.322%   $18.6455   $18.65   $18.65   $18.60  
  Purchase #3 $142.53   6.151%   $151.297   $151.30   $151.30   $151.30  
  Purchase #4 $226.61   4.112%   $235.9282   $235.93   $235.95   $235.90  
  Purchase #5 $98.73   8.395%   $107.0184   $107.02   $107.00   $107.00  
  Purchase #6 $191.46   6.687%   $204.2629   $204.26   $204.25   $204.30  
  Purchase #7 $24.66   11.670%   $27.5378   $27.54   $27.55   $27.50  
  7 purchase totals = $750.21       $801.1737   $801.18   $801.20   $801.10