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 $58.86   12.109%   $65.9874   $65.99   $66.00   $66.00  
  Purchase #2 $84.40   6.657%   $90.0185   $90.02   $90.00   $90.00  
  Purchase #3 $8.84   7.309%   $9.4861   $9.49   $9.50   $9.50  
  Purchase #4 $130.68   11.691%   $145.9578   $145.96   $145.95   $146.00  
  Purchase #5 $239.93   6.677%   $255.9501   $255.95   $255.95   $256.00  
  Purchase #6 $165.81   11.479%   $184.8433   $184.84   $184.85   $184.80  
  Purchase #7 $248.95   5.767%   $263.3069   $263.31   $263.30   $263.30  
  7 purchase totals = $937.47       $1,015.5501   $1,015.56   $1,015.55   $1,015.60