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 $100.60   7.519%   $108.1641   $108.16   $108.15   $108.20  
  Purchase #2 $89.76   10.423%   $99.1157   $99.12   $99.10   $99.10  
  Purchase #3 $223.35   6.422%   $237.6935   $237.69   $237.70   $237.70  
  Purchase #4 $215.54   8.536%   $233.9385   $233.94   $233.95   $233.90  
  Purchase #5 $157.89   6.350%   $167.916   $167.92   $167.90   $167.90  
  Purchase #6 $178.38   6.416%   $189.8249   $189.82   $189.80   $189.80  
  Purchase #7 $218.60   9.790%   $240.0009   $240.00   $240.00   $240.00  
  7 purchase totals = $1,184.12       $1,276.6536   $1,276.65   $1,276.60   $1,276.60