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 $31.86   8.381%   $34.5302   $34.53   $34.55   $34.50  
  Purchase #2 $47.79   10.563%   $52.8381   $52.84   $52.85   $52.80  
  Purchase #3 $60.67   5.145%   $63.7915   $63.79   $63.80   $63.80  
  Purchase #4 $49.31   10.197%   $54.3381   $54.34   $54.35   $54.30  
  Purchase #5 $212.71   13.810%   $242.0853   $242.09   $242.10   $242.10  
  Purchase #6 $197.59   8.569%   $214.5215   $214.52   $214.50   $214.50  
  Purchase #7 $150.35   7.741%   $161.9886   $161.99   $162.00   $162.00  
  7 purchase totals = $750.28       $824.0933   $824.10   $824.15   $824.00