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 $238.75   8.568%   $259.2061   $259.21   $259.20   $259.20  
  Purchase #2 $99.68   13.622%   $113.2584   $113.26   $113.25   $113.30  
  Purchase #3 $128.33   12.507%   $144.3802   $144.38   $144.40   $144.40  
  Purchase #4 $9.71   6.364%   $10.3279   $10.33   $10.35   $10.30  
  Purchase #5 $193.42   6.794%   $206.561   $206.56   $206.55   $206.60  
  Purchase #6 $11.51   5.737%   $12.1703   $12.17   $12.15   $12.20  
  Purchase #7 $167.58   8.609%   $182.007   $182.01   $182.00   $182.00  
  7 purchase totals = $848.98       $927.9109   $927.92   $927.90   $928.00