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 $12.31   4.719%   $12.8909   $12.89   $12.90   $12.90  
  Purchase #2 $112.75   5.776%   $119.2624   $119.26   $119.25   $119.30  
  Purchase #3 $97.68   3.482%   $101.0812   $101.08   $101.10   $101.10  
  Purchase #4 $37.97   3.173%   $39.1748   $39.17   $39.15   $39.20  
  Purchase #5 $41.75   7.633%   $44.9368   $44.94   $44.95   $44.90  
  Purchase #6 $71.30   9.345%   $77.963   $77.96   $77.95   $78.00  
  Purchase #7 $172.99   10.482%   $191.1228   $191.12   $191.10   $191.10  
  7 purchase totals = $546.75       $586.4319   $586.42   $586.40   $586.50