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 $126.67   9.751%   $139.0216   $139.02   $139.00   $139.00  
  Purchase #2 $102.79   12.343%   $115.4774   $115.48   $115.50   $115.50  
  Purchase #3 $50.69   13.519%   $57.5428   $57.54   $57.55   $57.50  
  Purchase #4 $116.59   8.320%   $126.2903   $126.29   $126.30   $126.30  
  Purchase #5 $151.58   4.590%   $158.5375   $158.54   $158.55   $158.50  
  Purchase #6 $244.95   9.864%   $269.1119   $269.11   $269.10   $269.10  
  Purchase #7 $87.60   9.404%   $95.8379   $95.84   $95.85   $95.80  
  7 purchase totals = $880.87       $961.8194   $961.82   $961.85   $961.70