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 $244.45   12.728%   $275.5636   $275.56   $275.55   $275.60  
  Purchase #2 $156.77   13.519%   $177.9637   $177.96   $177.95   $178.00  
  Purchase #3 $83.54   11.822%   $93.4161   $93.42   $93.40   $93.40  
  Purchase #4 $50.34   6.293%   $53.5079   $53.51   $53.50   $53.50  
  Purchase #5 $248.25   6.760%   $265.0317   $265.03   $265.05   $265.00  
  Purchase #6 $121.31   6.616%   $129.3359   $129.34   $129.35   $129.30  
  Purchase #7 $53.84   9.493%   $58.951   $58.95   $58.95   $59.00  
  7 purchase totals = $958.50       $1,053.7699   $1,053.77   $1,053.75   $1,053.80