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 $20.29   5.255%   $21.3562   $21.36   $21.35   $21.40  
  Purchase #2 $39.44   5.116%   $41.4578   $41.46   $41.45   $41.50  
  Purchase #3 $243.68   3.491%   $252.1869   $252.19   $252.20   $252.20  
  Purchase #4 $180.80   7.167%   $193.7579   $193.76   $193.75   $193.80  
  Purchase #5 $167.31   12.216%   $187.7486   $187.75   $187.75   $187.70  
  Purchase #6 $21.84   7.331%   $23.4411   $23.44   $23.45   $23.40  
  Purchase #7 $180.82   4.473%   $188.9081   $188.91   $188.90   $188.90  
  7 purchase totals = $854.18       $908.8566   $908.87   $908.85   $908.90