74LS90N Is not counting!
You should ALWAYS tie unused inputs somewhere.

This is especially true of control pins, but is also true for other inputs. Obviously, if you do not tie the control pins appropriately the device will not act as you expect. Tying other unused general inputs, although not required to ensure functionality, prevents noise in the system from causing unnecessary switching internal to the device and additional unwanted power consumption and potentially more EMI radiation.

Which way you tie them depends on the function of the pin.. 

For example .. if it is an "Enable" pin you need to tie it to the rail that enables it. However, if it is say a "Clear" pin you need to tie it to the rail that does not clear it. Check out the devices truth table to determine what to tie where.

You can tie the unused pins directly to the rail, however it is preferred to use appropriately sized pull-up or pull-down resistors. If you have multiple pins to tie, you can share a resistor for this, however if the design is new, and on a PCB, it is often prudent to use individual resistors for each pin. If you need to use that pin later in rework, the latter will save you a lot of grief. If it is tied directly to the rail, and the PCB has a ground plane, rework becomes a nightmare involving lifting the pin during manufacture.

Once the board becomes a mature product, you can remove the extra resistors on the final revision of the PCB as a cost reduction step.

Example imaginary device.....



The resistor sizes need to be small enough to ensure the input is pulled to the required logic voltage with sufficient current to be insensitive to noise, while not incurring too much wasted current. If using a single resistor for many pins, that should include the current for all pins attached to it. 

CMOS devices are easier to handle since they are symmetrical and have much lower currents. TTL on the other hand requires a much smaller resistor for pull-downs than pull-ups. That's why we generally pull-up when we have a choice.

Rule of thumb resistor math ...

\$R_{pulldown} = V_{IL}/(2*I_{IL})\$ For a single pin on an LS90 that's ~\$150R\$

\$R_{pullup} = (Vcc-V_{IH})/(2*I_{IH})\$ For a single pin on an LS90 that's ~\$18K\$. 

For TTL, 20K pull-ups is ok but, 10K is more common. I think we used to buy the latter by the ton.