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 $48.94   13.721%   $55.6551   $55.66   $55.65   $55.70  
  Purchase #2 $90.52   8.572%   $98.2794   $98.28   $98.30   $98.30  
  Purchase #3 $181.73   3.667%   $188.394   $188.39   $188.40   $188.40  
  Purchase #4 $57.78   11.594%   $64.479   $64.48   $64.50   $64.50  
  Purchase #5 $202.78   9.360%   $221.7602   $221.76   $221.75   $221.80  
  Purchase #6 $54.96   9.154%   $59.991   $59.99   $60.00   $60.00  
  Purchase #7 $117.77   9.175%   $128.5754   $128.58   $128.60   $128.60  
  7 purchase totals = $754.48       $817.1341   $817.14   $817.20   $817.30