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 $115.78   4.335%   $120.7991   $120.80   $120.80   $120.80  
  Purchase #2 $71.66   11.867%   $80.1639   $80.16   $80.15   $80.20  
  Purchase #3 $89.37   8.376%   $96.8556   $96.86   $96.85   $96.90  
  Purchase #4 $117.83   3.135%   $121.524   $121.52   $121.50   $121.50  
  Purchase #5 $66.33   11.224%   $73.7749   $73.77   $73.75   $73.80  
  Purchase #6 $147.54   9.327%   $161.3011   $161.30   $161.30   $161.30  
  Purchase #7 $8.41   13.728%   $9.5645   $9.56   $9.55   $9.60  
  7 purchase totals = $616.92       $663.9831   $663.97   $663.90   $664.10