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 $38.62   10.110%   $42.5245   $42.52   $42.50   $42.50  
  Purchase #2 $26.91   4.172%   $28.0327   $28.03   $28.05   $28.00  
  Purchase #3 $23.76   9.127%   $25.9286   $25.93   $25.95   $25.90  
  Purchase #4 $35.50   10.798%   $39.3333   $39.33   $39.35   $39.30  
  Purchase #5 $11.87   8.225%   $12.8463   $12.85   $12.85   $12.80  
  Purchase #6 $97.38   13.376%   $110.4055   $110.41   $110.40   $110.40  
  Purchase #7 $151.34   12.567%   $170.3589   $170.36   $170.35   $170.40  
  7 purchase totals = $385.38       $429.4298   $429.43   $429.45   $429.30