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 $80.71   10.375%   $89.0837   $89.08   $89.10   $89.10  
  Purchase #2 $14.75   7.116%   $15.7996   $15.80   $15.80   $15.80  
  Purchase #3 $172.51   6.813%   $184.2631   $184.26   $184.25   $184.30  
  Purchase #4 $127.71   3.794%   $132.5553   $132.56   $132.55   $132.60  
  Purchase #5 $64.79   5.353%   $68.2582   $68.26   $68.25   $68.30  
  Purchase #6 $79.58   4.531%   $83.1858   $83.19   $83.20   $83.20  
  Purchase #7 $6.41   7.375%   $6.8827   $6.88   $6.90   $6.90  
  7 purchase totals = $546.46       $580.0284   $580.03   $580.05   $580.20