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 $60.53   6.731%   $64.6043   $64.60   $64.60   $64.60  
  Purchase #2 $171.35   10.793%   $189.8438   $189.84   $189.85   $189.80  
  Purchase #3 $80.97   7.406%   $86.9666   $86.97   $86.95   $87.00  
  Purchase #4 $18.32   6.863%   $19.5773   $19.58   $19.60   $19.60  
  Purchase #5 $210.35   8.154%   $227.5019   $227.50   $227.50   $227.50  
  Purchase #6 $160.61   7.106%   $172.0229   $172.02   $172.00   $172.00  
  Purchase #7 $141.32   3.493%   $146.2563   $146.26   $146.25   $146.30  
  7 purchase totals = $843.45       $906.7731   $906.77   $906.75   $906.80