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 $100.68   4.649%   $105.3606   $105.36   $105.35   $105.40  
  Purchase #2 $12.49   10.570%   $13.8102   $13.81   $13.80   $13.80  
  Purchase #3 $244.52   10.440%   $270.0479   $270.05   $270.05   $270.00  
  Purchase #4 $241.34   4.147%   $251.3484   $251.35   $251.35   $251.30  
  Purchase #5 $234.57   3.538%   $242.8691   $242.87   $242.85   $242.90  
  Purchase #6 $37.32   5.800%   $39.4846   $39.48   $39.50   $39.50  
  Purchase #7 $14.67   13.383%   $16.6333   $16.63   $16.65   $16.60  
  7 purchase totals = $885.59       $939.5541   $939.55   $939.55   $939.50