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 $16.75   6.623%   $17.8594   $17.86   $17.85   $17.90  
  Purchase #2 $18.68   4.843%   $19.5847   $19.58   $19.60   $19.60  
  Purchase #3 $45.72   5.178%   $48.0874   $48.09   $48.10   $48.10  
  Purchase #4 $126.96   9.412%   $138.9095   $138.91   $138.90   $138.90  
  Purchase #5 $138.58   9.270%   $151.4264   $151.43   $151.45   $151.40  
  Purchase #6 $170.54   11.265%   $189.7513   $189.75   $189.75   $189.80  
  Purchase #7 $80.94   13.242%   $91.6581   $91.66   $91.65   $91.70  
  7 purchase totals = $598.17       $657.2768   $657.28   $657.30   $657.40