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 $70.38   9.746%   $77.2392   $77.24   $77.25   $77.20  
  Purchase #2 $15.82   11.572%   $17.6507   $17.65   $17.65   $17.70  
  Purchase #3 $167.29   8.799%   $182.0098   $182.01   $182.00   $182.00  
  Purchase #4 $91.30   6.522%   $97.2546   $97.25   $97.25   $97.30  
  Purchase #5 $71.86   4.346%   $74.983   $74.98   $75.00   $75.00  
  Purchase #6 $23.80   11.556%   $26.5503   $26.55   $26.55   $26.60  
  Purchase #7 $36.28   3.337%   $37.4907   $37.49   $37.50   $37.50  
  7 purchase totals = $476.73       $513.1783   $513.17   $513.20   $513.30