Dont get me wrong, I like Tony Allen. I am glad we resigned him. He is a very effective perimeter defender. He is able to stay with most SG's and PG's defensively, but SF? Keep in mind he is 6'4" in shoes at best.
I made a list of guys who play the wing/SF full or part time every nite that Tony may be matched up against at any given time. Some nites he may have to play 30+ minutes if Paul Pierce is hurting or we have foul problems. These are guys he will be expected to guard on the wing as well as SG's etc. Feel free to add a few names, I am sure I forgot some.
Lebron James
Kevin Durant
Lamar Odom
Shawn Marion
Josh Howard
Gerald Wallace
Richard Jefferson
Shane Battier
Tracy McGrady
Peja Stoyakovich
Rudy Gay
Josh Smith
Tayshawn Prince
Ron Artest
Out of all of those, this are the ones that should give him trouble....
LeBron James
Lamar Odom (he's been playing mostly PF though, but with Bynum coming he might get the SF nod)
Shawn Marion (an athletic freak)
Josh Howard (can have a nice post up game against shorter players)
Rudy Gay (tall and athletic, can give him trouble penetrating... he plays quite a bit of a perimeter game, so that plays in Tony's favor)
Tayshawn Prince (very long and can post him up easily)
The rest:
Josh Smith (He plays VERY little SF to be considered a problem)
Tracy McGrady (he plays mostly as a big SG, so I don't think it's relevant to this discussion)
Gerald Wallace (I'm pretty sure he won't play SF much after his latest concusion, I think he expressed as much)
Kevin Durant (still hasn't polished his game enough for me to worry about him at the moment. If he develops a better post up game, then he can give Tony trouble... else, his success will have little to do with Tony's deficiency guarding him)
Richard Jefferson (great at drawing the foul, that'll be his advantage. Can get 3's happy, and if Tony can close his driving lanes, he won't be an issue.)
Shane Battier (not enough on an offensive arsenal to really take advantage of a smaller defender)
Peja Stoyakovich (too much of a perimeter player to worry about him)
Ron Artest (his offensive game won't take advantage of a smaller defender, I'm not worried about him... Tony should be capable of guarding him).
That's my look at things. No doubt that Tony will have to work hard, but I can see him defending himself quite a bit against the great majority of the bunch.
With all that said, I think Giddens will get the SF nod over Tony. He's lengthier and taller. I'm pretty sure he's also stronger.