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 $79.48   8.464%   $86.2072   $86.21   $86.20   $86.20  
  Purchase #2 $182.57   5.639%   $192.8651   $192.87   $192.85   $192.90  
  Purchase #3 $11.66   5.756%   $12.3311   $12.33   $12.35   $12.30  
  Purchase #4 $176.38   12.632%   $198.6603   $198.66   $198.65   $198.70  
  Purchase #5 $24.63   6.580%   $26.2507   $26.25   $26.25   $26.30  
  Purchase #6 $238.90   5.562%   $252.1876   $252.19   $252.20   $252.20  
  Purchase #7 $217.27   5.709%   $229.6739   $229.67   $229.65   $229.70  
  7 purchase totals = $930.89       $998.1759   $998.18   $998.15   $998.30