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 $248.43   10.369%   $274.1897   $274.19   $274.20   $274.20  
  Purchase #2 $193.44   9.865%   $212.5229   $212.52   $212.50   $212.50  
  Purchase #3 $41.77   7.578%   $44.9353   $44.94   $44.95   $44.90  
  Purchase #4 $124.85   3.421%   $129.1211   $129.12   $129.10   $129.10  
  Purchase #5 $130.66   9.191%   $142.669   $142.67   $142.65   $142.70  
  Purchase #6 $65.81   11.781%   $73.5631   $73.56   $73.55   $73.60  
  Purchase #7 $23.79   11.727%   $26.5799   $26.58   $26.60   $26.60  
  7 purchase totals = $828.75       $903.5810   $903.58   $903.55   $903.60