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 $7.99   13.744%   $9.0881   $9.09   $9.10   $9.10  
  Purchase #2 $237.62   11.707%   $265.4382   $265.44   $265.45   $265.40  
  Purchase #3 $142.64   3.461%   $147.5768   $147.58   $147.60   $147.60  
  Purchase #4 $148.64   11.629%   $165.9253   $165.93   $165.95   $165.90  
  Purchase #5 $199.38   10.649%   $220.612   $220.61   $220.60   $220.60  
  Purchase #6 $102.56   4.604%   $107.2819   $107.28   $107.30   $107.30  
  Purchase #7 $198.87   6.499%   $211.7946   $211.79   $211.80   $211.80  
  7 purchase totals = $1,037.70       $1,127.7169   $1,127.72   $1,127.80   $1,127.70