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 $239.54   7.387%   $257.2348   $257.23   $257.25   $257.20  
  Purchase #2 $182.71   8.822%   $198.8287   $198.83   $198.85   $198.80  
  Purchase #3 $41.78   9.477%   $45.7395   $45.74   $45.75   $45.70  
  Purchase #4 $242.95   11.843%   $271.7226   $271.72   $271.70   $271.70  
  Purchase #5 $115.52   6.764%   $123.3338   $123.33   $123.35   $123.30  
  Purchase #6 $9.43   4.692%   $9.8725   $9.87   $9.85   $9.90  
  Purchase #7 $92.83   6.178%   $98.565   $98.57   $98.55   $98.60  
  7 purchase totals = $924.76       $1,005.2969   $1,005.29   $1,005.30   $1,005.20