Just want to know your opinion about Maxell DVD DL blank media which ImgBurn identifies as Ritek.
Ritek DVD+R DL are generally quite poor, especially on the second layer. A pack of Ritek DVD+R DL tends to have at least a 10% bad burn rate (1 bad disc, minimum, out of a 10-pack), and sometimes the failed burn is only detected after some testings -- the disc doesn't always kicked out of the drive as a failed burn. Additionally, the Ritek DL discs read quite poorly, as compared to Verbatim media.
You'd do better to buy and use the Mitsubishi discs sold under the Verbatim brand.
What's the exact media ID on the Ritek disc you have? Single-layer Ritek all moved to Fuji Oxonol dyes, and those have been excellent media. I'm still hoping that Ritek adopts Fuji methods for their DL discs, which theoretically would make them decent Mitsubishi substitutes, when on a lower budget.
Be sure to read the media guide: Blank DVD Media Quality Review
Additionally I've heard that keeping a disc (any disc) in the drive all the time can prevent the lens from getting dirty from dust, smoke etc. Is it true ?
No, that's not true at all.
The only way to keep dust out a DVD player is to seal it in a large ziplock bag -- but even that's not good, because condensation would build up.
... and welcome to the site.