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 $250.98   13.704%   $285.3743   $285.37   $285.35   $285.40  
  Purchase #2 $127.65   10.649%   $141.2434   $141.24   $141.25   $141.20  
  Purchase #3 $131.47   8.504%   $142.6502   $142.65   $142.65   $142.70  
  Purchase #4 $27.96   4.765%   $29.2923   $29.29   $29.30   $29.30  
  Purchase #5 $191.46   3.735%   $198.611   $198.61   $198.60   $198.60  
  Purchase #6 $74.73   13.108%   $84.5256   $84.53   $84.55   $84.50  
  Purchase #7 $79.30   11.346%   $88.2974   $88.30   $88.30   $88.30  
  7 purchase totals = $883.55       $969.9942   $969.99   $970.00   $970.00