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 $243.65   11.309%   $271.2044   $271.20   $271.20   $271.20  
  Purchase #2 $235.82   13.493%   $267.6392   $267.64   $267.65   $267.60  
  Purchase #3 $16.99   7.694%   $18.2972   $18.30   $18.30   $18.30  
  Purchase #4 $144.58   3.109%   $149.075   $149.08   $149.10   $149.10  
  Purchase #5 $183.28   12.568%   $206.3146   $206.31   $206.30   $206.30  
  Purchase #6 $69.93   8.616%   $75.9552   $75.96   $75.95   $76.00  
  Purchase #7 $240.29   12.142%   $269.466   $269.47   $269.45   $269.50  
  7 purchase totals = $1,134.54       $1,257.9516   $1,257.96   $1,257.95   $1,258.00