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 $234.54   13.428%   $266.034   $266.03   $266.05   $266.00  
  Purchase #2 $245.37   7.804%   $264.5187   $264.52   $264.50   $264.50  
  Purchase #3 $50.61   12.711%   $57.043   $57.04   $57.05   $57.00  
  Purchase #4 $200.94   5.179%   $211.3467   $211.35   $211.35   $211.30  
  Purchase #5 $197.84   11.853%   $221.29   $221.29   $221.30   $221.30  
  Purchase #6 $225.96   8.508%   $245.1847   $245.18   $245.20   $245.20  
  Purchase #7 $12.83   11.797%   $14.3436   $14.34   $14.35   $14.30  
  7 purchase totals = $1,168.09       $1,279.7607   $1,279.75   $1,279.80   $1,279.60