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 $84.72   8.147%   $91.6221   $91.62   $91.60   $91.60  
  Purchase #2 $203.93   10.692%   $225.7342   $225.73   $225.75   $225.70  
  Purchase #3 $18.96   11.113%   $21.067   $21.07   $21.05   $21.10  
  Purchase #4 $181.81   11.239%   $202.2436   $202.24   $202.25   $202.20  
  Purchase #5 $89.59   8.678%   $97.3646   $97.36   $97.35   $97.40  
  Purchase #6 $233.30   10.411%   $257.5889   $257.59   $257.60   $257.60  
  Purchase #7 $29.92   7.284%   $32.0994   $32.10   $32.10   $32.10  
  7 purchase totals = $842.23       $927.7198   $927.71   $927.70   $927.70