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 $50.41   11.566%   $56.2404   $56.24   $56.25   $56.20  
  Purchase #2 $227.70   8.612%   $247.3095   $247.31   $247.30   $247.30  
  Purchase #3 $247.96   8.421%   $268.8407   $268.84   $268.85   $268.80  
  Purchase #4 $29.38   3.575%   $30.4303   $30.43   $30.45   $30.40  
  Purchase #5 $5.35   4.586%   $5.5954   $5.60   $5.60   $5.60  
  Purchase #6 $92.30   10.705%   $102.1807   $102.18   $102.20   $102.20  
  Purchase #7 $47.48   5.392%   $50.0401   $50.04   $50.05   $50.00  
  7 purchase totals = $700.58       $760.6371   $760.64   $760.70   $760.50