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 $113.55   6.778%   $121.2464   $121.25   $121.25   $121.20  
  Purchase #2 $69.67   11.368%   $77.5901   $77.59   $77.60   $77.60  
  Purchase #3 $79.70   10.767%   $88.2813   $88.28   $88.30   $88.30  
  Purchase #4 $209.93   12.449%   $236.0642   $236.06   $236.05   $236.10  
  Purchase #5 $117.71   9.767%   $129.2067   $129.21   $129.20   $129.20  
  Purchase #6 $77.59   9.301%   $84.8066   $84.81   $84.80   $84.80  
  Purchase #7 $91.87   6.712%   $98.0363   $98.04   $98.05   $98.00  
  7 purchase totals = $760.02       $835.2316   $835.24   $835.25   $835.20