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 $34.33   12.561%   $38.6422   $38.64   $38.65   $38.60  
  Purchase #2 $71.69   3.726%   $74.3612   $74.36   $74.35   $74.40  
  Purchase #3 $138.55   11.343%   $154.2657   $154.27   $154.25   $154.30  
  Purchase #4 $157.78   6.339%   $167.7817   $167.78   $167.80   $167.80  
  Purchase #5 $223.48   8.616%   $242.735   $242.74   $242.75   $242.70  
  Purchase #6 $54.27   6.394%   $57.74   $57.74   $57.75   $57.70  
  Purchase #7 $167.74   4.722%   $175.6607   $175.66   $175.65   $175.70  
  7 purchase totals = $847.84       $911.1865   $911.19   $911.20   $911.20