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 $94.31   12.376%   $105.9818   $105.98   $106.00   $106.00  
  Purchase #2 $58.84   8.737%   $63.9809   $63.98   $64.00   $64.00  
  Purchase #3 $167.25   11.613%   $186.6727   $186.67   $186.65   $186.70  
  Purchase #4 $35.65   13.264%   $40.3786   $40.38   $40.40   $40.40  
  Purchase #5 $200.74   8.849%   $218.5035   $218.50   $218.50   $218.50  
  Purchase #6 $18.44   11.667%   $20.5914   $20.59   $20.60   $20.60  
  Purchase #7 $30.86   13.585%   $35.0523   $35.05   $35.05   $35.10  
  7 purchase totals = $606.09       $671.1612   $671.15   $671.20   $671.30