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 $28.40   7.723%   $30.5933   $30.59   $30.60   $30.60  
  Purchase #2 $38.95   12.625%   $43.8674   $43.87   $43.85   $43.90  
  Purchase #3 $84.63   8.212%   $91.5798   $91.58   $91.60   $91.60  
  Purchase #4 $44.45   12.586%   $50.0445   $50.04   $50.05   $50.00  
  Purchase #5 $213.80   3.437%   $221.1483   $221.15   $221.15   $221.10  
  Purchase #6 $81.32   9.484%   $89.0324   $89.03   $89.05   $89.00  
  Purchase #7 $125.82   3.285%   $129.9532   $129.95   $129.95   $130.00  
  7 purchase totals = $617.37       $656.2189   $656.21   $656.25   $656.20