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 $110.42   12.313%   $124.016   $124.02   $124.00   $124.00  
  Purchase #2 $5.30   7.869%   $5.7171   $5.72   $5.70   $5.70  
  Purchase #3 $98.39   13.582%   $111.7533   $111.75   $111.75   $111.80  
  Purchase #4 $221.27   8.807%   $240.7572   $240.76   $240.75   $240.80  
  Purchase #5 $173.51   3.304%   $179.2428   $179.24   $179.25   $179.20  
  Purchase #6 $168.72   8.512%   $183.0814   $183.08   $183.10   $183.10  
  Purchase #7 $192.37   9.509%   $210.6625   $210.66   $210.65   $210.70  
  7 purchase totals = $969.98       $1,055.2303   $1,055.23   $1,055.20   $1,055.30