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 $247.25   11.823%   $276.4824   $276.48   $276.50   $276.50  
  Purchase #2 $111.57   4.810%   $116.9365   $116.94   $116.95   $116.90  
  Purchase #3 $88.53   9.192%   $96.6677   $96.67   $96.65   $96.70  
  Purchase #4 $27.75   4.706%   $29.0559   $29.06   $29.05   $29.10  
  Purchase #5 $34.67   3.793%   $35.985   $35.99   $36.00   $36.00  
  Purchase #6 $172.95   12.672%   $194.8662   $194.87   $194.85   $194.90  
  Purchase #7 $26.60   12.613%   $29.9551   $29.96   $29.95   $30.00  
  7 purchase totals = $709.32       $779.9488   $779.97   $779.95   $780.10