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 $68.97   12.130%   $77.3361   $77.34   $77.35   $77.30  
  Purchase #2 $180.62   12.274%   $202.7893   $202.79   $202.80   $202.80  
  Purchase #3 $171.78   12.551%   $193.3401   $193.34   $193.35   $193.30  
  Purchase #4 $110.56   8.457%   $119.9101   $119.91   $119.90   $119.90  
  Purchase #5 $225.67   9.554%   $247.2305   $247.23   $247.25   $247.20  
  Purchase #6 $99.98   4.164%   $104.1432   $104.14   $104.15   $104.10  
  Purchase #7 $35.30   4.723%   $36.9672   $36.97   $36.95   $37.00  
  7 purchase totals = $892.88       $981.7165   $981.72   $981.75   $981.60