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 $228.28   5.115%   $239.9565   $239.96   $239.95   $240.00  
  Purchase #2 $104.52   12.297%   $117.3728   $117.37   $117.35   $117.40  
  Purchase #3 $29.39   9.281%   $32.1177   $32.12   $32.10   $32.10  
  Purchase #4 $103.69   13.766%   $117.964   $117.96   $117.95   $118.00  
  Purchase #5 $182.56   13.185%   $206.6305   $206.63   $206.65   $206.60  
  Purchase #6 $165.58   11.787%   $185.0969   $185.10   $185.10   $185.10  
  Purchase #7 $115.93   10.557%   $128.1687   $128.17   $128.15   $128.20  
  7 purchase totals = $929.95       $1,027.3071   $1,027.31   $1,027.25   $1,027.40