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 $8.79   12.185%   $9.8611   $9.86   $9.85   $9.90  
  Purchase #2 $200.96   9.591%   $220.2341   $220.23   $220.25   $220.20  
  Purchase #3 $6.60   9.744%   $7.2431   $7.24   $7.25   $7.20  
  Purchase #4 $200.32   11.166%   $222.6877   $222.69   $222.70   $222.70  
  Purchase #5 $164.71   4.286%   $171.7695   $171.77   $171.75   $171.80  
  Purchase #6 $183.28   7.580%   $197.1726   $197.17   $197.15   $197.20  
  Purchase #7 $92.60   9.153%   $101.0757   $101.08   $101.10   $101.10  
  7 purchase totals = $857.26       $930.0438   $930.04   $930.05   $930.10