Would you avoid a 24 hour urine collection for creatinine clearance measurement in a patient who is on fluid restriction?   

I don't think there is a reason to avoid a 24-hour urine collection in this setting. The excretion of creatinine in a 24-hour period is independent of urine flow rate.

No. The creatinine clearance should still be accurate if done correctly. The only problem may be if the urine output is lower, minor mistakes in urine collection will lead to bigger inaccuracies.