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 $211.70   3.128%   $218.322   $218.32   $218.30   $218.30  
  Purchase #2 $78.89   12.539%   $88.782   $88.78   $88.80   $88.80  
  Purchase #3 $71.61   5.188%   $75.3251   $75.33   $75.35   $75.30  
  Purchase #4 $51.89   4.508%   $54.2292   $54.23   $54.25   $54.20  
  Purchase #5 $35.92   3.236%   $37.0824   $37.08   $37.10   $37.10  
  Purchase #6 $115.91   12.394%   $130.2759   $130.28   $130.30   $130.30  
  Purchase #7 $30.49   12.701%   $34.3625   $34.36   $34.35   $34.40  
  7 purchase totals = $596.41       $638.3791   $638.38   $638.45   $638.40