Used to do it too because my arms used to feel heavy or something when I solved at comps. Doesn't really make a difference for me in terms of tps, but I ended up changing to either off the table or in my lap if it's 4x4+
Matěj Mužátko - 8.02 3x3 NR single - Brno Open 2015
U2 F' L' D L' D L2 U2 B R L D' B2 R2 D B2 D R2
x2 // inspection
U2 L' l' U2 l F' // Xcross
y U2' R' U R U' y R U R' // 2nd pair
L' U L R' U2 R U' R' U R // 3rd pair
L' U L y' U R U R' // 4th pair
U' f' L' U' L U f // OLL
U M2' U' M' U2' M U' M2' // PLL View at alg.cubing.net
x2 // inspection
U U L' l' U U l F' // Xcross
y U' U' R' U R U' y R U R' // 2nd pair
L' U L R' U U R U' R' U R // 3rd pair
L' U L y' U R U R' // 4th pair
U' f' L' U' L U f // OLL
U M2' U' M' U' U' M U' M2' // PLL View at alg.cubing.net