It is well defined law that declaration is enough to absolve the employer (deducting authority) from penalties which could otherwise amount to a jail term.
Once the declaration is given the onus falls on employee to furnish proof of investment failing which he will stand liable for penalties etc.
Now it is obviously taken for granted that an employee will provide Rent receipt only if he is paid HRA as component of salary.
Likewise for conveyance no declaration is needed as an amount of 800 pm is deductible.
PF again is deducted by the employer so the amount is known more to the employer than the employee (especially where mid term increments take effect).
Again for Medical- employee has to provide proof irrespective of whether he gets anything in lieu of this as part of salary.
Again an employee will make investments in tax saving bonds LIC etc only when he has to provide for IT liabilities.
So no employee will do anything on his own without having to allocate for tax liabilities on account of components of salary which are taxable.