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 $121.69   13.608%   $138.2496   $138.25   $138.25   $138.20  
  Purchase #2 $224.84   12.304%   $252.5043   $252.50   $252.50   $252.50  
  Purchase #3 $247.85   12.442%   $278.6875   $278.69   $278.70   $278.70  
  Purchase #4 $60.97   12.232%   $68.4279   $68.43   $68.45   $68.40  
  Purchase #5 $49.28   6.445%   $52.4561   $52.46   $52.45   $52.50  
  Purchase #6 $183.48   11.391%   $204.3802   $204.38   $204.40   $204.40  
  Purchase #7 $163.80   9.218%   $178.8991   $178.90   $178.90   $178.90  
  7 purchase totals = $1,051.91       $1,173.6047   $1,173.61   $1,173.65   $1,173.60