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 $134.69   3.450%   $139.3368   $139.34   $139.35   $139.30  
  Purchase #2 $121.40   9.781%   $133.2741   $133.27   $133.25   $133.30  
  Purchase #3 $149.99   7.772%   $161.6472   $161.65   $161.65   $161.60  
  Purchase #4 $104.41   9.269%   $114.0878   $114.09   $114.10   $114.10  
  Purchase #5 $38.44   7.835%   $41.4518   $41.45   $41.45   $41.50  
  Purchase #6 $31.27   4.444%   $32.6596   $32.66   $32.65   $32.70  
  Purchase #7 $158.56   13.715%   $180.3065   $180.31   $180.30   $180.30  
  7 purchase totals = $738.76       $802.7638   $802.77   $802.75   $802.80