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 $245.96   11.144%   $273.3698   $273.37   $273.35   $273.40  
  Purchase #2 $113.63   7.269%   $121.8898   $121.89   $121.90   $121.90  
  Purchase #3 $63.92   3.327%   $66.0466   $66.05   $66.05   $66.00  
  Purchase #4 $39.43   8.519%   $42.789   $42.79   $42.80   $42.80  
  Purchase #5 $250.58   8.668%   $272.3003   $272.30   $272.30   $272.30  
  Purchase #6 $120.79   6.371%   $128.4855   $128.49   $128.50   $128.50  
  Purchase #7 $89.82   6.493%   $95.652   $95.65   $95.65   $95.70  
  7 purchase totals = $924.13       $1,000.5330   $1,000.54   $1,000.55   $1,000.60