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 $118.45   13.367%   $134.2832   $134.28   $134.30   $134.30  
  Purchase #2 $86.72   13.276%   $98.2329   $98.23   $98.25   $98.20  
  Purchase #3 $45.27   3.442%   $46.8282   $46.83   $46.85   $46.80  
  Purchase #4 $185.27   11.728%   $206.9985   $207.00   $207.00   $207.00  
  Purchase #5 $202.95   4.838%   $212.7687   $212.77   $212.75   $212.80  
  Purchase #6 $190.89   3.805%   $198.1534   $198.15   $198.15   $198.20  
  Purchase #7 $97.82   8.828%   $106.4555   $106.46   $106.45   $106.50  
  7 purchase totals = $927.37       $1,003.7204   $1,003.72   $1,003.75   $1,003.80