Oh yeah, I've always put between 5-15% into TSP, never less than 5%. It's been closer to 5 the last few years as it was tough to get that $10K into the Roth for the wife and I while she was in school and not working. Yes, I do get the matching.
If you are lucky enough to not have any tax-deferreed (i.e. pre-tax money in any IRA accounts), there is an end around which is fairly easy, following the steps below:
1) Put $5000 post-tax money into an IRA account.
2) Immediately transfer that money into a Roth-IRA through a distribution with your broker.
3) Assuming you performed this immediately, there is no tax implication as you have no gains. If you do have gains, you will have to pay taxes on those gains, but this will likely be less than $5.
While there is an AGI limit on direct contributions to a Roth-IRA, you can contribute the max into an IRA (the limitation with an IRA is only that you cannot later deduct that money unless your AGI is below a certain threshold). Currently, there is no AGI limit on converting an IRA to a Roth-IRA.
I have done this the past two years, and I now have a Roth-IRA with $15k in contributions despite my income being above the AGI limit for a Roth-IRA.
Good Luck!