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 $228.84   12.345%   $257.0903   $257.09   $257.10   $257.10  
  Purchase #2 $26.28   10.279%   $28.9813   $28.98   $29.00   $29.00  
  Purchase #3 $26.94   8.582%   $29.252   $29.25   $29.25   $29.30  
  Purchase #4 $10.40   7.645%   $11.1951   $11.20   $11.20   $11.20  
  Purchase #5 $231.95   8.747%   $252.2387   $252.24   $252.25   $252.20  
  Purchase #6 $235.64   7.404%   $253.0868   $253.09   $253.10   $253.10  
  Purchase #7 $155.98   6.156%   $165.5821   $165.58   $165.60   $165.60  
  7 purchase totals = $916.03       $997.4263   $997.43   $997.50   $997.50