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 $200.74   11.872%   $224.5719   $224.57   $224.55   $224.60  
  Purchase #2 $119.32   13.366%   $135.2683   $135.27   $135.25   $135.30  
  Purchase #3 $222.59   7.874%   $240.1167   $240.12   $240.10   $240.10  
  Purchase #4 $97.76   4.113%   $101.7809   $101.78   $101.80   $101.80  
  Purchase #5 $66.80   3.669%   $69.2509   $69.25   $69.25   $69.30  
  Purchase #6 $216.95   12.264%   $243.5567   $243.56   $243.55   $243.60  
  Purchase #7 $244.99   10.791%   $271.4269   $271.43   $271.45   $271.40  
  7 purchase totals = $1,169.15       $1,285.9723   $1,285.98   $1,285.95   $1,286.10