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 $236.34   8.116%   $255.5214   $255.52   $255.50   $255.50  
  Purchase #2 $234.75   9.874%   $257.9292   $257.93   $257.95   $257.90  
  Purchase #3 $98.55   6.142%   $104.6029   $104.60   $104.60   $104.60  
  Purchase #4 $212.68   5.812%   $225.041   $225.04   $225.05   $225.00  
  Purchase #5 $248.70   12.176%   $278.9817   $278.98   $279.00   $279.00  
  Purchase #6 $172.37   5.268%   $181.4505   $181.45   $181.45   $181.50  
  Purchase #7 $73.62   4.279%   $76.7702   $76.77   $76.75   $76.80  
  7 purchase totals = $1,277.01       $1,380.2969   $1,380.29   $1,380.30   $1,380.30