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 $27.61   7.850%   $29.7774   $29.78   $29.80   $29.80  
  Purchase #2 $150.71   3.524%   $156.021   $156.02   $156.00   $156.00  
  Purchase #3 $126.89   9.415%   $138.8367   $138.84   $138.85   $138.80  
  Purchase #4 $111.84   13.552%   $126.9966   $127.00   $127.00   $127.00  
  Purchase #5 $36.44   3.418%   $37.6855   $37.69   $37.70   $37.70  
  Purchase #6 $22.87   10.710%   $25.3194   $25.32   $25.30   $25.30  
  Purchase #7 $118.93   8.230%   $128.7179   $128.72   $128.70   $128.70  
  7 purchase totals = $595.29       $643.3545   $643.37   $643.35   $643.30