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 $103.35   11.777%   $115.5215   $115.52   $115.50   $115.50  
  Purchase #2 $34.79   13.696%   $39.5548   $39.55   $39.55   $39.60  
  Purchase #3 $219.39   3.736%   $227.5864   $227.59   $227.60   $227.60  
  Purchase #4 $63.84   7.751%   $68.7882   $68.79   $68.80   $68.80  
  Purchase #5 $177.45   3.588%   $183.8169   $183.82   $183.80   $183.80  
  Purchase #6 $48.64   8.110%   $52.5847   $52.58   $52.60   $52.60  
  Purchase #7 $131.80   12.757%   $148.6137   $148.61   $148.60   $148.60  
  7 purchase totals = $779.26       $836.4662   $836.46   $836.45   $836.50