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 $133.39   5.562%   $140.8092   $140.81   $140.80   $140.80  
  Purchase #2 $22.28   9.655%   $24.4311   $24.43   $24.45   $24.40  
  Purchase #3 $92.64   13.799%   $105.4234   $105.42   $105.40   $105.40  
  Purchase #4 $75.66   6.832%   $80.8291   $80.83   $80.85   $80.80  
  Purchase #5 $60.72   4.618%   $63.524   $63.52   $63.50   $63.50  
  Purchase #6 $80.37   13.537%   $91.2497   $91.25   $91.25   $91.20  
  Purchase #7 $21.95   6.398%   $23.3544   $23.35   $23.35   $23.40  
  7 purchase totals = $487.01       $529.6209   $529.61   $529.60   $529.50