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 $39.38   6.601%   $41.9795   $41.98   $42.00   $42.00  
  Purchase #2 $37.88   9.432%   $41.4528   $41.45   $41.45   $41.50  
  Purchase #3 $32.68   8.232%   $35.3702   $35.37   $35.35   $35.40  
  Purchase #4 $29.38   11.789%   $32.8436   $32.84   $32.85   $32.80  
  Purchase #5 $170.50   4.862%   $178.7897   $178.79   $178.80   $178.80  
  Purchase #6 $35.92   9.253%   $39.2437   $39.24   $39.25   $39.20  
  Purchase #7 $149.87   13.524%   $170.1384   $170.14   $170.15   $170.10  
  7 purchase totals = $495.61       $539.8179   $539.81   $539.85   $539.80