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 $180.94   13.661%   $205.6582   $205.66   $205.65   $205.70  
  Purchase #2 $28.79   11.184%   $32.0099   $32.01   $32.00   $32.00  
  Purchase #3 $29.58   4.601%   $30.941   $30.94   $30.95   $30.90  
  Purchase #4 $8.30   5.521%   $8.7582   $8.76   $8.75   $8.80  
  Purchase #5 $125.61   7.843%   $135.4616   $135.46   $135.45   $135.50  
  Purchase #6 $199.75   5.623%   $210.9819   $210.98   $211.00   $211.00  
  Purchase #7 $77.78   11.752%   $86.9207   $86.92   $86.90   $86.90  
  7 purchase totals = $650.75       $710.7315   $710.73   $710.70   $710.80