Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $28.43   9.416%   $31.107   $31.11   $31.10   $31.10  
  Purchase #2 $7.59   3.312%   $7.8414   $7.84   $7.85   $7.80  
  Purchase #3 $164.43   9.490%   $180.0344   $180.03   $180.05   $180.00  
  Purchase #4 $107.43   3.183%   $110.8495   $110.85   $110.85   $110.80  
  Purchase #5 $186.51   10.263%   $205.6515   $205.65   $205.65   $205.70  
  Purchase #6 $145.97   12.735%   $164.5593   $164.56   $164.55   $164.60  
  Purchase #7 $98.92   12.221%   $111.009   $111.01   $111.00   $111.00  
  7 purchase totals = $739.28       $811.0521   $811.05   $811.05   $811.00