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 $121.90   10.815%   $135.0835   $135.08   $135.10   $135.10  
  Purchase #2 $115.96   10.792%   $128.4744   $128.47   $128.45   $128.50  
  Purchase #3 $171.69   6.818%   $183.3958   $183.40   $183.40   $183.40  
  Purchase #4 $42.30   10.659%   $46.8088   $46.81   $46.80   $46.80  
  Purchase #5 $235.25   3.534%   $243.5637   $243.56   $243.55   $243.60  
  Purchase #6 $76.25   3.308%   $78.7724   $78.77   $78.75   $78.80  
  Purchase #7 $61.48   9.844%   $67.5321   $67.53   $67.55   $67.50  
  7 purchase totals = $824.83       $883.6307   $883.62   $883.60   $883.70