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 $25.56   13.411%   $28.9879   $28.99   $29.00   $29.00  
  Purchase #2 $185.26   13.789%   $210.8055   $210.81   $210.80   $210.80  
  Purchase #3 $173.46   13.694%   $197.2136   $197.21   $197.20   $197.20  
  Purchase #4 $142.77   5.636%   $150.8165   $150.82   $150.80   $150.80  
  Purchase #5 $209.72   10.219%   $231.1513   $231.15   $231.15   $231.20  
  Purchase #6 $211.67   10.761%   $234.4478   $234.45   $234.45   $234.40  
  Purchase #7 $213.49   9.768%   $234.3437   $234.34   $234.35   $234.30  
  7 purchase totals = $1,161.93       $1,287.7663   $1,287.77   $1,287.75   $1,287.70