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 $40.70   11.486%   $45.3748   $45.37   $45.35   $45.40  
  Purchase #2 $20.91   7.637%   $22.5069   $22.51   $22.50   $22.50  
  Purchase #3 $220.46   7.277%   $236.5029   $236.50   $236.50   $236.50  
  Purchase #4 $142.97   3.206%   $147.5536   $147.55   $147.55   $147.60  
  Purchase #5 $37.91   4.656%   $39.6751   $39.68   $39.70   $39.70  
  Purchase #6 $9.33   3.824%   $9.6868   $9.69   $9.70   $9.70  
  Purchase #7 $45.57   9.276%   $49.7971   $49.80   $49.80   $49.80  
  7 purchase totals = $517.85       $551.0972   $551.10   $551.10   $551.20