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 $234.57   13.757%   $266.8398   $266.84   $266.85   $266.80  
  Purchase #2 $34.84   11.853%   $38.9696   $38.97   $38.95   $39.00  
  Purchase #3 $14.90   4.469%   $15.5659   $15.57   $15.55   $15.60  
  Purchase #4 $81.97   8.270%   $88.7489   $88.75   $88.75   $88.70  
  Purchase #5 $202.68   6.595%   $216.0467   $216.05   $216.05   $216.00  
  Purchase #6 $67.94   12.344%   $76.3265   $76.33   $76.35   $76.30  
  Purchase #7 $57.60   7.352%   $61.8348   $61.83   $61.85   $61.80  
  7 purchase totals = $694.50       $764.3322   $764.34   $764.35   $764.20