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 $28.38   9.658%   $31.1209   $31.12   $31.10   $31.10  
  Purchase #2 $27.45   10.838%   $30.425   $30.43   $30.45   $30.40  
  Purchase #3 $208.77   10.765%   $231.2441   $231.24   $231.25   $231.20  
  Purchase #4 $173.50   7.643%   $186.7606   $186.76   $186.75   $186.80  
  Purchase #5 $60.60   4.848%   $63.5379   $63.54   $63.55   $63.50  
  Purchase #6 $45.36   6.255%   $48.1973   $48.20   $48.20   $48.20  
  Purchase #7 $234.58   9.418%   $256.6727   $256.67   $256.65   $256.70  
  7 purchase totals = $778.64       $847.9585   $847.96   $847.95   $847.90