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 $71.80   11.530%   $80.0785   $80.08   $80.10   $80.10  
  Purchase #2 $194.33   7.672%   $209.239   $209.24   $209.25   $209.20  
  Purchase #3 $65.58   7.453%   $70.4677   $70.47   $70.45   $70.50  
  Purchase #4 $227.94   10.299%   $251.4155   $251.42   $251.40   $251.40  
  Purchase #5 $7.33   11.663%   $8.1849   $8.18   $8.20   $8.20  
  Purchase #6 $109.74   9.212%   $119.8492   $119.85   $119.85   $119.80  
  Purchase #7 $161.44   5.872%   $170.9198   $170.92   $170.90   $170.90  
  7 purchase totals = $838.16       $910.1546   $910.16   $910.15   $910.10