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 $121.90   6.464%   $129.7796   $129.78   $129.80   $129.80  
  Purchase #2 $170.48   9.799%   $187.1853   $187.19   $187.20   $187.20  
  Purchase #3 $5.81   8.142%   $6.2831   $6.28   $6.30   $6.30  
  Purchase #4 $136.86   13.430%   $155.2403   $155.24   $155.25   $155.20  
  Purchase #5 $47.92   7.323%   $51.4292   $51.43   $51.45   $51.40  
  Purchase #6 $209.75   12.217%   $235.3752   $235.38   $235.40   $235.40  
  Purchase #7 $129.61   10.136%   $142.7473   $142.75   $142.75   $142.70  
  7 purchase totals = $822.33       $908.0400   $908.05   $908.15   $908.00