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 $56.36   6.201%   $59.8549   $59.85   $59.85   $59.90  
  Purchase #2 $212.35   6.825%   $226.8429   $226.84   $226.85   $226.80  
  Purchase #3 $237.40   8.208%   $256.8858   $256.89   $256.90   $256.90  
  Purchase #4 $60.59   10.508%   $66.9568   $66.96   $66.95   $67.00  
  Purchase #5 $30.55   13.523%   $34.6813   $34.68   $34.70   $34.70  
  Purchase #6 $23.74   9.782%   $26.0622   $26.06   $26.05   $26.10  
  Purchase #7 $30.45   11.572%   $33.9737   $33.97   $33.95   $34.00  
  7 purchase totals = $651.44       $705.2576   $705.25   $705.25   $705.40