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 $58.58   13.439%   $66.4526   $66.45   $66.45   $66.50  
  Purchase #2 $208.59   7.172%   $223.5501   $223.55   $223.55   $223.60  
  Purchase #3 $106.53   12.193%   $119.5192   $119.52   $119.50   $119.50  
  Purchase #4 $105.97   8.747%   $115.2392   $115.24   $115.25   $115.20  
  Purchase #5 $176.76   12.559%   $198.9593   $198.96   $198.95   $199.00  
  Purchase #6 $227.58   4.446%   $237.6982   $237.70   $237.70   $237.70  
  Purchase #7 $72.33   9.705%   $79.3496   $79.35   $79.35   $79.30  
  7 purchase totals = $956.34       $1,040.7682   $1,040.77   $1,040.75   $1,040.80