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 $249.31   6.807%   $266.2805   $266.28   $266.30   $266.30  
  Purchase #2 $92.69   9.801%   $101.7745   $101.77   $101.75   $101.80  
  Purchase #3 $44.89   6.432%   $47.7773   $47.78   $47.80   $47.80  
  Purchase #4 $151.54   10.413%   $167.3199   $167.32   $167.30   $167.30  
  Purchase #5 $172.99   6.449%   $184.1461   $184.15   $184.15   $184.10  
  Purchase #6 $235.74   9.275%   $257.6049   $257.60   $257.60   $257.60  
  Purchase #7 $16.87   7.761%   $18.1793   $18.18   $18.20   $18.20  
  7 purchase totals = $964.03       $1,043.0825   $1,043.08   $1,043.10   $1,043.10