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 $249.41   3.146%   $257.2564   $257.26   $257.25   $257.30  
  Purchase #2 $21.94   9.340%   $23.9892   $23.99   $24.00   $24.00  
  Purchase #3 $244.62   12.352%   $274.8355   $274.84   $274.85   $274.80  
  Purchase #4 $111.93   8.415%   $121.3489   $121.35   $121.35   $121.30  
  Purchase #5 $97.26   3.776%   $100.9325   $100.93   $100.95   $100.90  
  Purchase #6 $69.57   7.123%   $74.5255   $74.53   $74.55   $74.50  
  Purchase #7 $41.26   8.804%   $44.8925   $44.89   $44.90   $44.90  
  7 purchase totals = $835.99       $897.7805   $897.79   $897.85   $897.70