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 $250.66   8.596%   $272.2067   $272.21   $272.20   $272.20  
  Purchase #2 $165.89   6.824%   $177.2103   $177.21   $177.20   $177.20  
  Purchase #3 $131.55   3.222%   $135.7885   $135.79   $135.80   $135.80  
  Purchase #4 $182.27   7.806%   $196.498   $196.50   $196.50   $196.50  
  Purchase #5 $128.36   13.765%   $146.0288   $146.03   $146.05   $146.00  
  Purchase #6 $182.63   4.846%   $191.4802   $191.48   $191.50   $191.50  
  Purchase #7 $47.47   8.542%   $51.5249   $51.52   $51.50   $51.50  
  7 purchase totals = $1,088.83       $1,170.7374   $1,170.74   $1,170.75   $1,170.70