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 $49.51   9.696%   $54.3105   $54.31   $54.30   $54.30  
  Purchase #2 $60.53   5.370%   $63.7805   $63.78   $63.80   $63.80  
  Purchase #3 $223.92   9.604%   $245.4253   $245.43   $245.45   $245.40  
  Purchase #4 $76.68   4.694%   $80.2794   $80.28   $80.30   $80.30  
  Purchase #5 $90.26   8.515%   $97.9456   $97.95   $97.95   $97.90  
  Purchase #6 $87.59   6.816%   $93.5601   $93.56   $93.55   $93.60  
  Purchase #7 $230.81   7.694%   $248.5685   $248.57   $248.55   $248.60  
  7 purchase totals = $819.30       $883.8699   $883.88   $883.90   $883.90