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 $250.77   10.450%   $276.9755   $276.98   $277.00   $277.00  
  Purchase #2 $24.43   12.757%   $27.5465   $27.55   $27.55   $27.50  
  Purchase #3 $46.58   6.596%   $49.6524   $49.65   $49.65   $49.70  
  Purchase #4 $91.45   3.340%   $94.5044   $94.50   $94.50   $94.50  
  Purchase #5 $184.35   11.721%   $205.9577   $205.96   $205.95   $206.00  
  Purchase #6 $214.36   8.864%   $233.3609   $233.36   $233.35   $233.40  
  Purchase #7 $134.45   5.339%   $141.6283   $141.63   $141.65   $141.60  
  7 purchase totals = $946.39       $1,029.6257   $1,029.63   $1,029.65   $1,029.70