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 $5.60   3.125%   $5.775   $5.78   $5.80   $5.80  
  Purchase #2 $123.76   12.280%   $138.9577   $138.96   $138.95   $139.00  
  Purchase #3 $185.82   10.287%   $204.9353   $204.94   $204.95   $204.90  
  Purchase #4 $26.80   10.176%   $29.5272   $29.53   $29.55   $29.50  
  Purchase #5 $182.89   4.579%   $191.2645   $191.26   $191.25   $191.30  
  Purchase #6 $61.52   12.608%   $69.2764   $69.28   $69.30   $69.30  
  Purchase #7 $94.78   12.357%   $106.492   $106.49   $106.50   $106.50  
  7 purchase totals = $681.17       $746.2281   $746.24   $746.30   $746.30