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 $6.44   7.116%   $6.8983   $6.90   $6.90   $6.90  
  Purchase #2 $5.52   11.427%   $6.1508   $6.15   $6.15   $6.20  
  Purchase #3 $15.73   8.667%   $17.0933   $17.09   $17.10   $17.10  
  Purchase #4 $84.58   5.366%   $89.1186   $89.12   $89.10   $89.10  
  Purchase #5 $235.48   12.457%   $264.8137   $264.81   $264.80   $264.80  
  Purchase #6 $96.78   3.486%   $100.1538   $100.15   $100.15   $100.20  
  Purchase #7 $28.77   11.331%   $32.0299   $32.03   $32.05   $32.00  
  7 purchase totals = $473.30       $516.2584   $516.25   $516.25   $516.30