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 $246.42   13.621%   $279.9849   $279.98   $280.00   $280.00  
  Purchase #2 $28.82   13.826%   $32.8047   $32.80   $32.80   $32.80  
  Purchase #3 $119.93   3.854%   $124.5521   $124.55   $124.55   $124.60  
  Purchase #4 $248.81   11.215%   $276.714   $276.71   $276.70   $276.70  
  Purchase #5 $166.67   13.295%   $188.8288   $188.83   $188.85   $188.80  
  Purchase #6 $240.26   10.760%   $266.112   $266.11   $266.10   $266.10  
  Purchase #7 $204.73   10.447%   $226.1181   $226.12   $226.10   $226.10  
  7 purchase totals = $1,255.64       $1,395.1146   $1,395.10   $1,395.10   $1,395.10