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 $157.33   8.338%   $170.4482   $170.45   $170.45   $170.40  
  Purchase #2 $139.29   10.474%   $153.8792   $153.88   $153.90   $153.90  
  Purchase #3 $86.81   6.742%   $92.6627   $92.66   $92.65   $92.70  
  Purchase #4 $81.35   7.103%   $87.1283   $87.13   $87.15   $87.10  
  Purchase #5 $143.99   7.262%   $154.4466   $154.45   $154.45   $154.40  
  Purchase #6 $215.30   3.789%   $223.4577   $223.46   $223.45   $223.50  
  Purchase #7 $104.53   3.753%   $108.453   $108.45   $108.45   $108.50  
  7 purchase totals = $928.60       $990.4757   $990.48   $990.50   $990.50