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 $124.32   11.650%   $138.8033   $138.80   $138.80   $138.80  
  Purchase #2 $240.93   13.805%   $274.1904   $274.19   $274.20   $274.20  
  Purchase #3 $110.49   4.647%   $115.6245   $115.62   $115.60   $115.60  
  Purchase #4 $187.52   3.252%   $193.6182   $193.62   $193.60   $193.60  
  Purchase #5 $121.25   8.361%   $131.3877   $131.39   $131.40   $131.40  
  Purchase #6 $233.66   5.102%   $245.5813   $245.58   $245.60   $245.60  
  Purchase #7 $25.41   8.746%   $27.6324   $27.63   $27.65   $27.60  
  7 purchase totals = $1,043.58       $1,126.8378   $1,126.83   $1,126.85   $1,126.80