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 $122.85   3.820%   $127.5429   $127.54   $127.55   $127.50  
  Purchase #2 $222.80   4.649%   $233.158   $233.16   $233.15   $233.20  
  Purchase #3 $187.31   6.832%   $200.107   $200.11   $200.10   $200.10  
  Purchase #4 $22.90   7.707%   $24.6649   $24.66   $24.65   $24.70  
  Purchase #5 $238.48   8.611%   $259.0155   $259.02   $259.00   $259.00  
  Purchase #6 $25.83   8.801%   $28.1033   $28.10   $28.10   $28.10  
  Purchase #7 $106.27   10.561%   $117.4932   $117.49   $117.50   $117.50  
  7 purchase totals = $926.44       $990.0848   $990.08   $990.05   $990.10