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 $27.55   4.353%   $28.7493   $28.75   $28.75   $28.70  
  Purchase #2 $121.99   6.646%   $130.0975   $130.10   $130.10   $130.10  
  Purchase #3 $77.26   3.511%   $79.9726   $79.97   $79.95   $80.00  
  Purchase #4 $57.68   8.135%   $62.3723   $62.37   $62.35   $62.40  
  Purchase #5 $45.67   7.108%   $48.9162   $48.92   $48.90   $48.90  
  Purchase #6 $142.47   9.525%   $156.0403   $156.04   $156.05   $156.00  
  Purchase #7 $192.38   13.231%   $217.8338   $217.83   $217.85   $217.80  
  7 purchase totals = $665.00       $723.9820   $723.98   $723.95   $723.90