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 $39.79   12.546%   $44.7821   $44.78   $44.80   $44.80  
  Purchase #2 $173.69   6.121%   $184.3216   $184.32   $184.30   $184.30  
  Purchase #3 $87.42   9.266%   $95.5203   $95.52   $95.50   $95.50  
  Purchase #4 $195.44   13.568%   $221.9573   $221.96   $221.95   $222.00  
  Purchase #5 $87.32   4.707%   $91.4302   $91.43   $91.45   $91.40  
  Purchase #6 $188.76   10.724%   $209.0026   $209.00   $209.00   $209.00  
  Purchase #7 $114.83   9.569%   $125.8181   $125.82   $125.80   $125.80  
  7 purchase totals = $887.25       $972.8322   $972.83   $972.80   $972.80