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 $203.55   13.611%   $231.2552   $231.26   $231.25   $231.30  
  Purchase #2 $115.57   11.712%   $129.1056   $129.11   $129.10   $129.10  
  Purchase #3 $237.85   12.227%   $266.9319   $266.93   $266.95   $266.90  
  Purchase #4 $173.78   7.523%   $186.8535   $186.85   $186.85   $186.90  
  Purchase #5 $138.77   9.276%   $151.6423   $151.64   $151.65   $151.60  
  Purchase #6 $179.56   13.784%   $204.3106   $204.31   $204.30   $204.30  
  Purchase #7 $75.61   13.513%   $85.8272   $85.83   $85.85   $85.80  
  7 purchase totals = $1,124.69       $1,255.9263   $1,255.93   $1,255.95   $1,255.90