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 $24.54   12.108%   $27.5113   $27.51   $27.50   $27.50  
  Purchase #2 $189.45   6.198%   $201.1921   $201.19   $201.20   $201.20  
  Purchase #3 $66.58   13.328%   $75.4538   $75.45   $75.45   $75.50  
  Purchase #4 $217.99   11.646%   $243.3771   $243.38   $243.40   $243.40  
  Purchase #5 $203.78   13.599%   $231.492   $231.49   $231.50   $231.50  
  Purchase #6 $56.34   11.448%   $62.7898   $62.79   $62.80   $62.80  
  Purchase #7 $150.61   13.760%   $171.3339   $171.33   $171.35   $171.30  
  7 purchase totals = $909.29       $1,013.1500   $1,013.14   $1,013.20   $1,013.20