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 $19.30   9.386%   $21.1115   $21.11   $21.10   $21.10  
  Purchase #2 $57.64   10.766%   $63.8455   $63.85   $63.85   $63.80  
  Purchase #3 $130.94   6.198%   $139.0557   $139.06   $139.05   $139.10  
  Purchase #4 $129.62   10.538%   $143.2794   $143.28   $143.30   $143.30  
  Purchase #5 $156.38   9.170%   $170.72   $170.72   $170.70   $170.70  
  Purchase #6 $14.68   8.715%   $15.9594   $15.96   $15.95   $16.00  
  Purchase #7 $138.38   10.834%   $153.3721   $153.37   $153.35   $153.40  
  7 purchase totals = $646.94       $707.3436   $707.35   $707.30   $707.40