There are a few Newegg rebate checks that I recently found, where I had forgotten to cash them and its been way past the usual validity printed on the check. I understand that honoring the 'valid until' signage is upto the issuing bank, but other than just taking my chances to deposit these, is there a better way to make sure I can get them deposited?
Your best bet would be to contact Newegg. A brief search on Google for "Can I cash an expired rebate check?" returned mixed results.
So your best bet is to contact Newegg - or just take them to your bank to deposit, the worst that can happen is the bank says they can't take it.
In general, for an old (expired) check, you should first try and contact the issuer (which may be a Rebate processing firm, not the retailer) to have them re-issue the check. If you can't find contact information for the rebate processor, the retailer can probably get you pointed in the right direction if they don't handle it themselves, though it wouldn't surprise me if you needed to get transferred around a bit to find somebody who can help you.
Another place to check, especially if it's been several years, is your state's Abandoned Property office. Most states require that abandoned property be turned over to them, and rebate checks in some circumstances qualify.