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 $86.81   12.589%   $97.7385   $97.74   $97.75   $97.70  
  Purchase #2 $85.79   11.396%   $95.5666   $95.57   $95.55   $95.60  
  Purchase #3 $235.74   9.409%   $257.9208   $257.92   $257.90   $257.90  
  Purchase #4 $21.84   13.424%   $24.7718   $24.77   $24.75   $24.80  
  Purchase #5 $224.50   13.479%   $254.7604   $254.76   $254.75   $254.80  
  Purchase #6 $151.76   7.858%   $163.6853   $163.69   $163.70   $163.70  
  Purchase #7 $33.79   5.760%   $35.7363   $35.74   $35.75   $35.70  
  7 purchase totals = $840.23       $930.1797   $930.19   $930.15   $930.20