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 $121.91   6.518%   $129.8561   $129.86   $129.85   $129.90  
  Purchase #2 $144.43   8.186%   $156.253   $156.25   $156.25   $156.30  
  Purchase #3 $89.36   8.487%   $96.944   $96.94   $96.95   $96.90  
  Purchase #4 $110.41   12.386%   $124.0854   $124.09   $124.10   $124.10  
  Purchase #5 $39.71   6.674%   $42.3602   $42.36   $42.35   $42.40  
  Purchase #6 $65.90   12.287%   $73.9971   $74.00   $74.00   $74.00  
  Purchase #7 $11.48   10.479%   $12.683   $12.68   $12.70   $12.70  
  7 purchase totals = $583.20       $636.1788   $636.18   $636.20   $636.30