Looking at the OP's situation, where the PC is going to be on 90% of the time, I don't think it will be worth it. I assume that by being one 90% of the time we are talking about on, but idle. In that case you are looking at power consumption way below the 20% mark that 80+ tests at. We are talking power consumption in the 100w or less range typically. Now the difference between Silver and Gold is only 2%. Yes, some PSUs start to really go crazy below 20% load and efficiency really drops on all of them, but you can assume that for the most part the 2% difference in efficiency stays about the same between a Silver and a Gold PSU.
So assuming your computer is sitting idle almost all the time, and it is on 90% of the time. That means in a 30 day month it is sitting idle about 650 hours. At 100w usage, the difference between Silver and Gold is a whole 2w.
650h x 2W = 1300 Wh
1300Wh / 1000 = 1.3 kWh
So in a given month you are saving 1.3 kWh by going with a Gold over a Silver. At $0.13 per kWh, that is a savings of a whole $0.17 per month, or about $2 per year.
It simply isn't worth it.
Now, the most important part we have to remember is that some Silver PSUs are actually rated as Gold but marketed as Silver. I know, sounds strange, right, but it is true. Corsairs HX850 and HX750(probably the most popular Silver units on the market) are actually rated as 80+ Gold. Corsair markets them as 80+ Silver so that they don't hurt the sales of their AX line, and fit better with the other units in the HX line.
Also, a unit might be 80+ Silver only because the 100% load number didn't meet the mark, or overall it was only .1% below 80+ Gold. It is an assumption that because a unit is 80+ Silver it is 2% less efficient than 80+ Gold. That isn't always case, some Silver units are very very close to 80+ Gold and the difference is only a fraction of a %.