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 $112.84   6.269%   $119.9139   $119.91   $119.90   $119.90  
  Purchase #2 $90.71   5.701%   $95.8814   $95.88   $95.90   $95.90  
  Purchase #3 $76.39   13.325%   $86.569   $86.57   $86.55   $86.60  
  Purchase #4 $150.94   7.295%   $161.9511   $161.95   $161.95   $162.00  
  Purchase #5 $135.50   11.684%   $151.3318   $151.33   $151.35   $151.30  
  Purchase #6 $34.44   10.438%   $38.0348   $38.03   $38.05   $38.00  
  Purchase #7 $97.37   13.431%   $110.4478   $110.45   $110.45   $110.40  
  7 purchase totals = $698.19       $764.1298   $764.12   $764.15   $764.10