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 $28.57   6.397%   $30.3976   $30.40   $30.40   $30.40  
  Purchase #2 $207.77   6.113%   $220.471   $220.47   $220.45   $220.50  
  Purchase #3 $212.30   13.573%   $241.1155   $241.12   $241.10   $241.10  
  Purchase #4 $154.56   12.874%   $174.4581   $174.46   $174.45   $174.50  
  Purchase #5 $90.72   13.715%   $103.1622   $103.16   $103.15   $103.20  
  Purchase #6 $108.66   11.487%   $121.1418   $121.14   $121.15   $121.10  
  Purchase #7 $167.37   7.157%   $179.3487   $179.35   $179.35   $179.30  
  7 purchase totals = $969.95       $1,070.0949   $1,070.10   $1,070.05   $1,070.10