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 $61.33   6.211%   $65.1392   $65.14   $65.15   $65.10  
  Purchase #2 $79.46   6.152%   $84.3484   $84.35   $84.35   $84.30  
  Purchase #3 $177.74   13.426%   $201.6034   $201.60   $201.60   $201.60  
  Purchase #4 $36.68   9.837%   $40.2882   $40.29   $40.30   $40.30  
  Purchase #5 $101.93   11.801%   $113.9588   $113.96   $113.95   $114.00  
  Purchase #6 $62.86   3.610%   $65.1292   $65.13   $65.15   $65.10  
  Purchase #7 $58.96   13.319%   $66.8129   $66.81   $66.80   $66.80  
  7 purchase totals = $578.96       $637.2801   $637.28   $637.30   $637.20