i want to use the via holes as seen in picture below 
.
is it safe to do so ? i want to use those pins for digital i/o and nothing else. if its safe do i solder to the top of the board or is it safe to solder at the bottom like i do for headers.
Which Arduino board is this ?
Many use pins 0 and 1 for their Serial interface, including uploading code, and trying to use them for anything else can cause problems
As to your original question, yes, it is safe in the electrical and mechanical sense to solder to the vias if done properly but the joint may need supporting for strain relief
Why do you want/need to use pins 0 and 1 as digital pins ?
this is the arduino leonardo. I am running out of pins,just need a single extra pin so using this pin would be helpful instead of buying a i/o expander which are very expensive including shipping where i live.
Then why not connect to the proper 0 and 1 pins rather than the vias ? How many of the A* pins have you used ?
Are you sure that the vias are actually connected to pins 0 and 1 on the Leonardo ?
Please provide the details of your pin connections. Very often, forum members have been able to point out some rearrangement that frees up some new pins to use.
i have used all of them,yeah the via's if i am not wrong are on digital pin 30 and 17 respectively which are rx,tx built in leds
...but how?
pins 5-13 (9 toggle switches),pins 4-1(4 momentary switches), pin0(one led tog switch),pins14,16(pushpull switches),pin 15(one momentary switch(simulating engine start)), pin 23-20 (rotary encoder), pin 18-19(key switch with 3 prongs(one prong is ground)). just need one pin now for a normal push on-off pin
It sounds like creative use of the Keypad library could help here or a resistor ladder on a single analogue pin
You seem to have more pins on your Leonardo than I have on mine 
There are 14 pins at the one side (from 0-13) and 6 analog pins at the other side.
building a euro truck button box
@sterretje I make that 19 pins, as used by @itcorekjyk, albeit he is referring to the analogue pins by their pin numbers not their A* names
Isn't that wrong for coding the Leonardo? On the UNO, pin 14-19 is A0-A5, but I think not on a Leo...
i could use a button matrix but then dont want to deal with all the code and getting all the didodes and all of that to work.
You are correct. Pins A0 to A5 have values of 18 to 23 on the Leo. So, the correct number of pins, just the wrong pin numbers
Personally I have never seen the attraction of using the pin numbers rather than their names
No "didodes" needed for a keyboard matrix
I suspected that
From post #8
5 - 13
4 - 1
0
That completes the 14 pins at the one side
14,16
15
23 - 20
18 - 19
That makes 9 pins. The analog pins A0 .. A5 are 18 - 23.
I have 14, 15 and 16 as unknown pins; possibly the SPI. So yes, OP is out of pins 
15 is sck,16 is mosi,14 is miso but using those means no icsp capability
But you are using them 
for non ghosting everyone seems to be using diodes. anyways getting the buttons to click and then off and then again click is time consuming.for normal non matrix i am just copy pasting code and making adjustments.