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 $218.75   4.426%   $228.4319   $228.43   $228.45   $228.40  
  Purchase #2 $110.61   12.265%   $124.1763   $124.18   $124.20   $124.20  
  Purchase #3 $12.82   11.429%   $14.2852   $14.29   $14.30   $14.30  
  Purchase #4 $44.72   5.422%   $47.1447   $47.14   $47.15   $47.10  
  Purchase #5 $130.64   9.140%   $142.5805   $142.58   $142.60   $142.60  
  Purchase #6 $29.50   7.781%   $31.7954   $31.80   $31.80   $31.80  
  Purchase #7 $45.42   9.699%   $49.8253   $49.83   $49.85   $49.80  
  7 purchase totals = $592.46       $638.2393   $638.25   $638.35   $638.20