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 $18.89   6.439%   $20.1063   $20.11   $20.10   $20.10  
  Purchase #2 $63.71   9.205%   $69.5745   $69.57   $69.55   $69.60  
  Purchase #3 $229.86   12.374%   $258.3029   $258.30   $258.30   $258.30  
  Purchase #4 $105.25   10.818%   $116.6359   $116.64   $116.65   $116.60  
  Purchase #5 $250.94   10.315%   $276.8245   $276.82   $276.80   $276.80  
  Purchase #6 $147.27   6.342%   $156.6099   $156.61   $156.60   $156.60  
  Purchase #7 $41.87   12.722%   $47.1967   $47.20   $47.20   $47.20  
  7 purchase totals = $857.79       $945.2507   $945.25   $945.20   $945.20