its still a debated subject. rule of thumb, the ester has to be acetate or propionate, or the substance has to be completely unestrified (injectable winstrol, testosterone suspension...). Short esters and no esters are the only types of injection that should be "spot injected"
the argument FOR spot injecting is that due to the low to no estrification the steroid molecule is very rapidly released, which gives the spot injected muscle the first crack at it along with all of the muscles receptors. = spot specific growth. also, it is theorized that the swelling caused by short ester irritation allows for greater growth of the muscle fascia, further than they would be able to grow if not irritated.
the argument AGAINST this is that spot injectiong only causes irritation and makes the muscle appear to be bigger....which just brings us back to 2nd point in the FOR argument. personally, i see spot specific growth even with testosterone propionate. tren ace worked particularly well for me in this regard. winstrol is a favorite but i cant vouch for it