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 $7.49   3.793%   $7.7741   $7.77   $7.75   $7.80  
  Purchase #2 $236.51   3.634%   $245.1048   $245.10   $245.10   $245.10  
  Purchase #3 $136.82   4.861%   $143.4708   $143.47   $143.45   $143.50  
  Purchase #4 $111.89   9.174%   $122.1548   $122.15   $122.15   $122.20  
  Purchase #5 $70.80   13.205%   $80.1491   $80.15   $80.15   $80.10  
  Purchase #6 $76.46   4.686%   $80.0429   $80.04   $80.05   $80.00  
  Purchase #7 $170.57   11.440%   $190.0832   $190.08   $190.10   $190.10  
  7 purchase totals = $810.54       $868.7797   $868.76   $868.75   $868.80