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 $73.38   10.701%   $81.2324   $81.23   $81.25   $81.20  
  Purchase #2 $183.93   8.652%   $199.8436   $199.84   $199.85   $199.80  
  Purchase #3 $32.83   11.407%   $36.5749   $36.57   $36.55   $36.60  
  Purchase #4 $45.85   7.587%   $49.3286   $49.33   $49.35   $49.30  
  Purchase #5 $229.83   5.399%   $242.2385   $242.24   $242.25   $242.20  
  Purchase #6 $16.94   7.467%   $18.2049   $18.20   $18.20   $18.20  
  Purchase #7 $158.28   13.362%   $179.4294   $179.43   $179.45   $179.40  
  7 purchase totals = $741.04       $806.8523   $806.84   $806.90   $806.70