Grimmauld Place's location was revealed to Yaxley when he held onto Hermione, while she was apparating there. This indicates that the Secret is vulnerable for the length of time it is being accessed by the Secret Keeper.
As we Disapparated, Yaxley caught hold of me and I couldn’t get rid of him, he was too strong, and he was still holding on when we arrived at Grimmauld Place, and then – well, I think he must have seen the door, and thought we were stopping there, so he slackened his grip and I managed to sake him off and I brought us here instead
-Hermione Granger, Harry Potter and the Deathly Hallows
I take this to mean that when the Secret Keeper touches the wall of Grimmauld Place from the outside or even gazes at it from the outside(outside the area that is protected by the Fidelius), Grimmauld Place becomes visible for that brief duration of time.
NOTE: This vulnerability applies only to the Secret Keeper, and not to a person who has simply been told the secret.
The best way for a Secret Keeper to enter would be to apparate directly inside, which has its drawbacks as demonstrated by Hermione.
Bill Weasley was the Secret Keeper for Shell Cottage. Would it not be dangerous since he was actually living there? Every time Bill went out and entered Shell Cottage, he was risking the Fidelius. James, not being his own Secret Keeper was a practical choice, even if it backfired. Why did Bill risk it?