There are only three validly tested and nationally certified field sobriety tests (FSTs). The Horizontal Gaze Nystagmus, the Walk and Turn and the One Leg Stand. These tests, properly administered and taken as a whole, have some scientific value such that they can be a valid basis for probable cause to arrest. However, if the tests are not properly administered, or other tests are subjected in their stead, they lose scientific validity such that the tests may become inadmissible. Other tests, like the Finger Dexterity, Alphabet, Romberg Balance, etc are just so many Poll Tests with no independent and scientifically reliable study supporting their validity.
The FSTs, even though arguably of scientific value, are still searches of your person. Law enforcement personnel may not search your person unless you are under arrest or give them consent to do so. Because the Officer does not yet know whether he can arrest you, the point of the FSTs is to give the Officer justification in arresting you. Therefore, the Officer cannot compel you to perform the tests. As such, he/she must secure your consent to the tests. In a Washington State Supreme Court case, State v. Ferrier, the Court ruled that before searching a home incident to a consensual search, the Officers must inform the home occupant of his/her right to refuse the search, to limit the search and to rescind consent at anytime. It has not been decided by the Washington State Supreme Court whether greater or less protection applies to the search of a person's body is more or less invasive than a search of their home. However, it is fairly universally accepted that some degree of consent must be obtained for the FSTs. Thusly, most Officers will tell you that they'd like you to perform "voluntary' FSTs. If the Officer did not at a minimum say the word voluntary then likely the search is not consensual, unlawful and thus suppressable.
What doe it mean that the FSTs are suppressable? That means that your defense attorney could make a motion to suppress the FSTs. If the Judge grants this motion then the FSTs are out and cannot be explained to the Jury. Further, as the FSTs are often the basis for the arrest, the arrest decision is also unsupported. That means all evidence gathered after the unsupported arrest, including incriminating statements and even the breath or blood alcohol test (BAC) result are also suppressed. Needless to say, without a BAC or FSTs the prosecution no longer has much evidence to present to a jury to obtain a DUI conviction.
In any event, because the tests are voluntary you should always decline to take them. If for no other reason, you have better things to do then stand in the cold/dark/rain/dangerous roadway. You're refusal to take the FSTs will force the Officer to make a decision on whether to arrest you or not based on the information he/she has already. He/She will always arrest you at this point. The Officer already thinks you are under the influence. That's why they asked you to perform the FSTs. Even if you did perform them, he/she was going to arrest you anyway. Either way, you are going to the station so you might as well speed up the process by refusing the FSTs. Further, it gives the Officer much less information to justify your arrest and the prosecution less evidence to show a jury that you were under the influence.