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 $11.88   13.751%   $13.5136   $13.51   $13.50   $13.50  
  Purchase #2 $226.39   13.191%   $256.2531   $256.25   $256.25   $256.30  
  Purchase #3 $208.80   9.557%   $228.755   $228.76   $228.75   $228.80  
  Purchase #4 $250.58   4.819%   $262.6555   $262.66   $262.65   $262.70  
  Purchase #5 $48.35   9.103%   $52.7513   $52.75   $52.75   $52.80  
  Purchase #6 $237.27   12.321%   $266.504   $266.50   $266.50   $266.50  
  Purchase #7 $77.76   3.152%   $80.211   $80.21   $80.20   $80.20  
  7 purchase totals = $1,061.03       $1,160.6435   $1,160.64   $1,160.60   $1,160.80