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 $250.49   3.634%   $259.5928   $259.59   $259.60   $259.60  
  Purchase #2 $186.38   4.374%   $194.5323   $194.53   $194.55   $194.50  
  Purchase #3 $48.65   10.399%   $53.7091   $53.71   $53.70   $53.70  
  Purchase #4 $236.73   11.812%   $264.6925   $264.69   $264.70   $264.70  
  Purchase #5 $185.77   12.287%   $208.5956   $208.60   $208.60   $208.60  
  Purchase #6 $145.60   10.230%   $160.4949   $160.49   $160.50   $160.50  
  Purchase #7 $163.45   12.686%   $184.1853   $184.19   $184.20   $184.20  
  7 purchase totals = $1,217.07       $1,325.8025   $1,325.80   $1,325.85   $1,325.80