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 $226.33   5.598%   $239   $239.00   $239.00   $239.00  
  Purchase #2 $87.75   13.793%   $99.8534   $99.85   $99.85   $99.90  
  Purchase #3 $88.31   6.797%   $94.3124   $94.31   $94.30   $94.30  
  Purchase #4 $229.63   12.185%   $257.6104   $257.61   $257.60   $257.60  
  Purchase #5 $28.56   6.348%   $30.373   $30.37   $30.35   $30.40  
  Purchase #6 $229.82   10.645%   $254.2843   $254.28   $254.30   $254.30  
  Purchase #7 $50.79   9.847%   $55.7913   $55.79   $55.80   $55.80  
  7 purchase totals = $941.19       $1,031.2248   $1,031.21   $1,031.20   $1,031.30