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 $27.33   4.231%   $28.4863   $28.49   $28.50   $28.50  
  Purchase #2 $79.76   7.652%   $85.8632   $85.86   $85.85   $85.90  
  Purchase #3 $97.95   9.354%   $107.1122   $107.11   $107.10   $107.10  
  Purchase #4 $107.50   3.401%   $111.1561   $111.16   $111.15   $111.20  
  Purchase #5 $80.71   4.178%   $84.0821   $84.08   $84.10   $84.10  
  Purchase #6 $183.33   3.780%   $190.2599   $190.26   $190.25   $190.30  
  Purchase #7 $161.37   7.553%   $173.5583   $173.56   $173.55   $173.60  
  7 purchase totals = $737.95       $780.5181   $780.52   $780.50   $780.70