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 $77.89   4.573%   $81.4519   $81.45   $81.45   $81.50  
  Purchase #2 $172.91   7.459%   $185.8074   $185.81   $185.80   $185.80  
  Purchase #3 $126.32   13.328%   $143.1559   $143.16   $143.15   $143.20  
  Purchase #4 $193.40   4.424%   $201.956   $201.96   $201.95   $202.00  
  Purchase #5 $75.28   8.821%   $81.9204   $81.92   $81.90   $81.90  
  Purchase #6 $241.53   12.824%   $272.5038   $272.50   $272.50   $272.50  
  Purchase #7 $85.97   8.705%   $93.4537   $93.45   $93.45   $93.50  
  7 purchase totals = $973.30       $1,060.2491   $1,060.25   $1,060.20   $1,060.40