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 $45.58   12.568%   $51.3085   $51.31   $51.30   $51.30  
  Purchase #2 $53.74   13.456%   $60.9713   $60.97   $60.95   $61.00  
  Purchase #3 $217.50   11.333%   $242.1493   $242.15   $242.15   $242.10  
  Purchase #4 $87.66   11.199%   $97.477   $97.48   $97.50   $97.50  
  Purchase #5 $117.25   8.481%   $127.194   $127.19   $127.20   $127.20  
  Purchase #6 $85.94   3.155%   $88.6514   $88.65   $88.65   $88.70  
  Purchase #7 $162.42   3.760%   $168.527   $168.53   $168.55   $168.50  
  7 purchase totals = $770.09       $836.2785   $836.28   $836.30   $836.30