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 $156.88   13.354%   $177.8298   $177.83   $177.85   $177.80  
  Purchase #2 $202.27   7.378%   $217.1935   $217.19   $217.20   $217.20  
  Purchase #3 $202.75   10.793%   $224.6328   $224.63   $224.65   $224.60  
  Purchase #4 $223.78   6.286%   $237.8468   $237.85   $237.85   $237.80  
  Purchase #5 $51.61   10.220%   $56.8845   $56.88   $56.90   $56.90  
  Purchase #6 $97.26   6.661%   $103.7385   $103.74   $103.75   $103.70  
  Purchase #7 $196.64   12.850%   $221.9082   $221.91   $221.90   $221.90  
  7 purchase totals = $1,131.19       $1,240.0341   $1,240.03   $1,240.10   $1,239.90