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 $28.50   5.255%   $29.9977   $30.00   $30.00   $30.00  
  Purchase #2 $245.82   13.333%   $278.5952   $278.60   $278.60   $278.60  
  Purchase #3 $178.27   6.738%   $190.2818   $190.28   $190.30   $190.30  
  Purchase #4 $72.79   7.269%   $78.0811   $78.08   $78.10   $78.10  
  Purchase #5 $202.91   7.818%   $218.7735   $218.77   $218.75   $218.80  
  Purchase #6 $94.37   12.856%   $106.5022   $106.50   $106.50   $106.50  
  Purchase #7 $236.33   7.323%   $253.6364   $253.64   $253.65   $253.60  
  7 purchase totals = $1,058.99       $1,155.8679   $1,155.87   $1,155.90   $1,155.90