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 $221.31   12.860%   $249.7705   $249.77   $249.75   $249.80  
  Purchase #2 $88.49   9.392%   $96.801   $96.80   $96.80   $96.80  
  Purchase #3 $21.93   6.809%   $23.4232   $23.42   $23.40   $23.40  
  Purchase #4 $25.52   10.854%   $28.2899   $28.29   $28.30   $28.30  
  Purchase #5 $212.35   11.599%   $236.9805   $236.98   $237.00   $237.00  
  Purchase #6 $244.82   12.279%   $274.8814   $274.88   $274.90   $274.90  
  Purchase #7 $162.39   13.801%   $184.8014   $184.80   $184.80   $184.80  
  7 purchase totals = $976.81       $1,094.9479   $1,094.94   $1,094.95   $1,095.00