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 $79.66   12.495%   $89.6135   $89.61   $89.60   $89.60  
  Purchase #2 $94.99   9.444%   $103.9609   $103.96   $103.95   $104.00  
  Purchase #3 $90.79   8.716%   $98.7033   $98.70   $98.70   $98.70  
  Purchase #4 $13.79   13.799%   $15.6929   $15.69   $15.70   $15.70  
  Purchase #5 $72.79   12.858%   $82.1493   $82.15   $82.15   $82.10  
  Purchase #6 $192.86   9.542%   $211.2627   $211.26   $211.25   $211.30  
  Purchase #7 $187.86   7.844%   $202.5957   $202.60   $202.60   $202.60  
  7 purchase totals = $732.74       $803.9783   $803.97   $803.95   $804.00