If you want to ROUND
the results, you can calculate your own value to use in TOP
(granted, this means that you need to first count the rows of your table, but it's the only way that I can think of doing this, since there isn't a setting for this):
DECLARE @TopPercent INT, @Top INT
SET @TopPercent = 10 -- use the value you want here
SELECT @Top = ROUND(COUNT(*)*CAST(@TopPercent AS DECIMAL(4,1))/100,0)
FROM #ClientsTable
SELECT TOP(@Top)
HospMastID,
ClientID,
ControlGroup = 1
INTO #RandomTable
FROM #ClientsTable
ORDER BY NEWID()